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Originally appearing in Volume V26, Page 907 of the 1911 Encyclopedia Britannica.
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THYROSTRACA, an order of Crustacea, comprising barnacles, acorn shells and some allied degenerate parasites. The embryos are free-swimming, active forms, but in adult life the animals are fixed head downwards, and are very degenerate. The body is indistinctly segmented, and is enveloped in a fold of the integument, usually with calcareous plates. The anterior antennae are fused with the anchoring attachment, whilst the posterior pair is vestigial, and the appendages of the mouth and body present various degrees of degeneration and specialization. In most cases the adults are hermaphrodite, but unisexual forms also occur, whilst the hermaphrodite adults may carry with them minute " complementary " males. In strong contrast with the condition in most Crustacea, the spermatozoa are mobile. As shown by Burmeister in his historical review (1834), these animals, comprised by Linnaeus in the genus Lepas, first received a more comprehensive title from Cuvier, who called them Cirrhopoda, a word strictly meaning tawny-footed. Lamarck in ',Soo altered this into the hybrid form Cirrhipoda, meaning curl-footed, which was subsequently improved into Cirripedia or Cirrhipedia. So long as the group was held to be a subordinate member of the Entomostraca, this term, though not the earliest, was generally accepted. The name Thyrostraca, meaning doorshells or valve-shells, is preferred as agreeing in termination with the titles of the other two divisions, the Malacostraca and Entomostraca. The group may conveniently be arranged in two principal sections—the Genuina with cirrhiform feet, and the Anomala without them. Thyrostraca genuina.—It is with these that Darwin's classical treatises (Ray Soc. and Palaeont. Soc., 1851-1854) are almost exclusively concerned. Therein an order Thoracica comprehends the pedunculate Lepadidae, together with the operculate and sessile Balanidae and Verrucidae; a single species without cirrhi constitutes the order Apoda, and a single species with only three pairs of cirrhi the order Abdominalia. Within the last Kochlorine (Noll, 1872) with two species, and Lithoglyptes (Aurivillius, 1892) with three species, have since been included. But H. J. Hansen (Die Cirripedien der Plankton-Expedition, 1899) states that Cryptophialus minutus, for which the order Abdominalia was founded, has, like Alcippe and other Genuina, its cirrhi on the thorax, not, as Darwin wrongly supposed, on the abdomen. In place, therefore, of the Abdominalia, it will be right to accept the family Cryptophialidae, v. Martens, side by side with the Lithoglyptidae of Aurivillius and the Alcippidae of Gerstaecker. These, with Darwin's three families above mentioned, complete the section of genuine cirripedes now existing. Gruvel submitted to the Linnaean Society a rearrangement of the Lepadidae, unfortunately using for the first of his new families the preoccupied name Anaspidae. It is con-fusing, but not uninstructive, to find that within the Balanid group such generic titles as Stephanolepas and Platylepas have been coined. The vernacular name barnacle, traceable to the fable of pedunculate cirripedes hatching out into bernicle geese, has also been transferred to the sessile cirripedes, which are popularly known as acorn barnacles. A complete list of all the recent species of Thyrostraca in both sections, down to the year 1897, was published by Weltner (Arch. Naturg., 1898, § 63, pt. i. pp. 227-280). For fossil species, Woodward's Catalogue of Brit. Foss. Crust. (1877), pp. 137-144, should be consulted. Hoek (" Challenger " Reports, Cirripedia," 1883, viii. 8-n), gives a brief mouth." Darwin notes B. improvisus as quite tolerant of water geological summary down to 1882. In that year J. M. Clarke (Amer. Journ. Sci. and Arts, 3rd series, vol. xxiv., p. 55) added a new species to Plumulites (Barrande, 1872), remarking that the species in question, P. devonicus, " is interesting in being the first representative of fossil barnacles from the Devonian, Barrande's species of Plumulites and Anatifopsis, as well as the Turrilepis of \Voodward, being from the Upper and Lower Silurian, and Plumulites jamesi (Hall and Whitf., Pal. Ohio, vol. ii.) from the Hudson River group." Since Plumulites appears to be a synonym of Turrilepas (not Turrilepis), the species Turrilepas wrightii (Woodward, 1865), from the Upper Silurian of Dudley, did not long enjoy an isolated eminence as the oldest known cirripede. As pointed out by Dr Bather (Geol. Mag. 1901, decade 4, vol. viii. p. 521), palaeontologists themselves have in this branch not very closely followed the progress of their own science, since Dr Ruedemann, in regard to his new Pollicipes siluricus, 1901, speaks of " the enormous gap existing between the appearance of this Lower Siluric type and the next Upper Triassic (Rhaetic) representatives of the genera Pollicipes and Scalpellum." Nlany species of Scalpellum from the Wenlock shale of Gotland were described in 1892 by C. W. S. Aurivillius, who at the same time founded the species Pollicipes signatus on an almost perfect specimen from the Lower Ludlow of Wisby in Sweden. Aurivillius considered that Pollicipes signatus showed a closer approach to the Balanidae than any other of the Lepadidae, but he, too, in ignorance of the Devonian Protobalanus (Whitf.), discoursed needlessly about the gap in the distribution. Dr Bather justifiably anticipates further discoveries, but if, already in Silurian as in modern times, the members of these families had to pass through nauplius and cypris stages to maturity, there is one " enormous gap " between them and the common ancestor of the crustacean class that will not be easily filled. To later phylogenetic links an addition is offered by Dr Wood-ward (Geol. Mag., 1901, p. 145), who transfers his Pyrgoma cretaceum, 1865, to a new genus Brachylepas (fig. 1), and 'a new family Brachylepadidae, intermediate between the Rhaetic Pollicipes and the modern Balanus. Among !" i , •y 1 other fossil genera of recent insti- l~ tution, Archaeolepas, Lepidocoleus, chalk search. search. Among living forms, added since Weltner's catalogue, may be noticed Koleolepas willeyi, from the Loyalty Islands (Stebbing, in Willey's Zool. Results (1900) pt. V., p. 677. This was found in a Turbo shell, occupied also by a Pagurid, and coated with Actinians. The cirripede, which has an elastic peduncle, a crested capitulum, but no valves, and the first cirrhi longer than the rest, should stand near Eremolepas, the name given by Weltner in place of the preoccupied Gymnolepas (Aurivillius). In the genus Scalpellum, S. giganteum, Gruvel (Trans. Linn. Soc., 1901) disputes with S. stearnsii (fig. 2), Pilsbury, 189o, which shall be accounted the greater. The latter is threatened with a new generic name (Chun, Aus den Tiefen des Weltmeeres, 1900, p. 502). The horizontal distribution of barnacles over all seas is fully explained by Darwin. The bathymetric range of sessile as well as pedunculate forms down to such depths as twelve or eighteen thousand feet—Verruca quadrangularis, Hoek, 1900 fathoms; Scalpellum regium, Wyville Thomson, 285o s ii. (Contribution a l'etude des Cirrhipedes, 1894) found that the species frequenting sea sur- face or shallow water, notwithstanding their feeble powers of vision, cannot live long when entirely debarred from light. It must, therefore, be supposed that abyssal forms have gradually acquired such tolerance of darkness as makes their health independent of the sun. Among other singularities of habitat, not the least curious is the freedom with which some small species, especially in the genus Dichelaspis, occupy the very jaws of large crustaceans. It is generally stated that cirripedes are confined to salt water, and, generally speaking, that is true. But Platylepas bissexlobata (De Blainville), from the west coast of Africa, is sometimes found entirely buried, except its operculum, in the skin of the manatee. Now, since it seems this Manatus senegalensis ascends rivers, we may infer that its parasite travels with it. Studer (Crustacea of the Gazelle, 1882) records Balanus amphitrite (Darwin?) from roots and stems of mangroves in the Congo, where, he says, " it follows the mangroves as far as their vegetation extends along the stream, to six sea-miles from the not saline. Why the Thyrostraca, so hardy, so widely dispersed and multitudinous, and with a history so prolonged, should not have made more extended and more independent incursions into fresh water remains a problem. Though the Ornitholepas australis (Targioni Tozzetti, 1872), found on the tail feathers of a bird, represents only the cypris-larva of a cirripede, it still shows one of the many facilities for dispersion which these creatures enjoy. A striking instance of their abundance is cited by Aurivillius (1894) from a report by Captain G. C. Eckman, who late in the summer observed great masses of Lepas fascicularis forming broad belts in the North Sea. Aurivillius himself examined a humpback whale which had as many as fifty specimens of Coronula diadema on each side of its head. He believes that the cetacean approaches not only rocks, but ships, in the hope of freeing itself from its lodgers. Yet the fact that the long, soft Conchoderma auritum stands exposed on the Coronula, sometimes ten on one, indicates that the whale can have little chance of evicting its tenants, even at the expense of rubbing off the eighteen flattened horns of its own skin embedded in cavities round the domed base of the Coronula shell. The fecundity in the genus Lepas has struck many observers. Hoek (" Challenger " Reports, Cirripedia," 1884, vol. x.) notes that, while in Scalpellum the number of eggs may be less than a hundred, " in Lepas anatifera it amounts, on the contrary, to many thousands and tens of thousands." In the same treatise Dr Hoek has useful chapters on the anatomy, development and sexes of the group, with references to the important researches since Darwin by Krohn, Claus, Kossmann and others. Francis Darwin, in the life of his father (1888, iii. 2), says, " Krohn stated that the structures described by my father as ovaries were in reality salivary glands, also that the oviduct runs down to the orifice described in the Mono-graph of the Ciryipedia as the auditory meatus." Hoek, however, observes that the interpretation of the glands as salivary is not given by Krohn as his own opinion, but only quoted from Cuvier. Hoek himself proposes to call them pancreatic glands. On the absorbing question of the development, T. T. Groom (Phil. Trans., 1894, vol. clxxxv.) supplies a full bibliography, beginning with the pioneers Slabber (1778; properly 1769) and J. Vaughan Thomson (183o). Groom's monograph was almost immediately supplemented by Chun's chapters on the same subject (Bibliotheca Zoologica, 1895, Heft 19, Lieferung 2), to which an important discussion is contributed by H. J. Hansen (Die Cirripedien der Plankton-Exp., 1899). He insists on the value of the upper lip or labrum for generic distinction, and as an aid in affiliating larval forms of different stages to their several species. He cites Groom's evidence that larvae obtained from the egg readily go through one moult in the aquarium, and the known fact that the last larval stage is marked by a longitudinal series of six pairs of immovable spines or processes. He considers, then, that by a judicious comparison of larval forms with these two easily determinable stages the poverty of existing information on the subject may be gradually, if laboriously, diminished. The large and peculiar Archizoea gigas of Dohrn must, he thinks, belong to the Lepadidae as a larva in the last stage, but not, as v. Willemoes Suhm supposed, to Lepas australis, or even to the genus Lepas at all. Thyrostraca anomala.—This section comprises Darwin's Apoda, the footless, Lilljeborg's Suctoria, called by Fritz Muller the Rhizocephala or rootlet-headed, and the group to which Lacaze-Duthiers gave the alternative names Ascothoracida, sac-bodied or Rhizothoracida, rootlet-bodied. For the present these names may be dispensed with in favour of their equivalents, the three families Proteolepadidae, Sacculinidae and Lauridae. The first is still limited to the single genus and species Proteolepas bivincta (Darwin), parasitic within the sac of another cirripede. Nothing is certainly known of its development, except that the ova are extremely small, but H. J. Hansen (Die Cirripedien der Plankton-Exp.,1899, p. 53) argues that various nauplii of a type not previously described may probably be referred to this group or family. The second family, discussed by Delage, Giard, Kossmann and others, has no dearth of genera and species, though about several of them the in-formation is scanty. Almost all of them are parasitic on other crustaceans. Sphaerothylacus polycarpus (Sluiter, 1884) has an ascidian for its host. Sarcotaces (Olsson, 1872) has two species parasitic in fishes. But these exceptional and dubious forms do not obtain nutriment by sending rootlets in a rhizocephalous manner into their patrons. The family Peltogastridae is sometimes separated from the Sacculinidae, and sometimes made to do duty for both, the latter course being improper, since Sacculina (J. Vaughan Thompson, 1836) is not, as has been supposed, preoccupied, and must, therefore, take precedence of Peltogaster (Rathke, 1843). In the same family stands the genus Sylon, noted by Kroyer with-out a name in 1842, named by him without a description in 1855, described by Michael Sars in 1869, and published by G. O. Sars in 187o. Hoek (" Challenger " Reports, 1888, vol. xxiv. app. A) will orientate the English reader on this genus. For the complicated parasitism of isopods and Sacculinidae on the same hosts Giard and Bonnier (Bopyriens, 1887, p. 197) should be consulted. The remaining family may, till further knowledge, be allowed to cover four remarkable species, three of them resident on Anthozoa, one on an echinoderm. Only the first, the celebrated Laura gerardiae (Lacaze-Duthiers, 1865), sends such rootlets into its host as would justify the term Rhizothoracida. The small sinuous segmented body is enclosed, except for one small opening, in an enormous sac-like carapace, between the lamellae of which are protruded from the body the ovary and " liver," both large, bifurcate and ramified. It is this sac-like and not valvular carapace, therefore, that justifies the term Ascothoracida. But Synagoga mira, Norman, 1888 (Brit. Assoc. Report for 1887), has the body covered by two nearly circular valves instead of a sac. Petrarca bathyactidis, G. H. Fowler (Quart. Journ. Mic. Sci., 1889, vol. xxx. pt. ii. p. 115), has a bilobed carapace, ventrally open; Dendrogaster astericola, Knipovitch (Biologisches Centralblatt, 1891, X. 707), is a multilobular sac, with apparently indistinct segmentation of the body proper on the dorsal side. For this highly problematic group the original authorities should by all means be consulted. The student may then be asked to compare the account of Synagoga mira both with the figure of the cyprisstage of Dendrogaster astericola and with the figure of the " indeterminate animal found on Gerardia," about which Lacaze-Duthiers asks, " Is it the cypris-stage of Laura?" (Mem. Acad. Sci., 1883, xlii. 16o, pl. 1, fig. 102). S. mira was found, like Laura gerardiae (fig. 3), in the Mediterranean, and found like it attached show the body proper. to an antipatharian. Its six pairs of limbs are not like the bare and simple feet of the Laura, but two-branched and setose as in the ordinary cypris-stage of the cirripede. The conclusion, therefore, from these facts and from the suggested comparisons, seems at least extremely probable that the question asked by H. Lacaze-Duthiers should be answered in the affirmative, and that S. mira is either the cypris-stage of Laura gerardiae or of some congeneric species. In Lacaze-Duthiers's highly-elaborated memoir it should be noticed that he uses the term " cirrhes " rather misleadingly, not for cirrhiform feet, but as the equivalent of setae. Also he gives two different reckonings of the segmentation, counting first eleven body segments without the caudal furca (p. 40), and then the caudal furca as itself the eleventh segment (p. 41). Of Petrarca the development is not yet known. The points of agreement and difference between it and Laura are carefully drawn out in the essay by Dr G. H. Fowler, who inclines to favour a close relation-ship between the Thyrostraca and Ostracoda. To the extreme development of the carapace in Laura, as compared with the segmented body, it would be difficult to find among crustaceans any analogy more striking than that of the great ovarial expansions in Nicothoe astaci, the little copepod parasite of the common lobster. The compactness of the class Crustacea is generally admitted; of the precise affinities of its subdivisions there is still much to learn. (T. R. R. S.)
End of Article: THYROSTRACA
THYROID (Gr. Ovpoea3i'is, shield-shaped, from Ovpeo...
THYRSUS (the latinized form of Gr. Bupaos, a stem o...

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