CRUSTACEA ago the Cirripedia were still arranged as Mollusca in many public museums ; nor is this surprising, considering the Fig. 82. Fig. 83. FTG. 82 Young of PeUogaster socialis on the abdomen of a small hermit-crab ; in one of them the fasciculately ramified roots, r, in the liver of the crab are shown. The animal and roots are deep yellow. (Fritz Miiller.) FIG. 83. Young of Sacculina pttrpurea with its roots ; the animal purple-red, the roots dark grass green. Magn. 5 diam. (Fritz Miiller.) fixed condition of their shells and the degree of external resemblance between Lepas and Teredo on the one hand, and Balanus and a compound of a Patella and a Chiton shell on the other. Strauss in 1819 first affirmed that Cirripedes were Crustacea. Bub this view was disregarded until J. Vaughan Thompson s capital discovery in 1830 of their metamorphosis, since which time they have been almost universally placed with the Crustacea. The Cirripedia are classed by Darwin in three great divisions: (1) THOEACICA (limbs thoracic) ; (2) ABDOMI- NALIA (limbs abdominal) ; (3) APODA (appendages want ing). In the first division are embraced the Balanidai, the Vermicides, and the Lepadidce ; in the second a single genus, Gryptophialus minutus ; the third is also repre sented by one form, the Proteolepas bivincta. (1) THOEACICA. Cirripedes ordinarily are bisexual, 1 in which respect they differ from all other Crustacea, the male (where it exists distinct) being minute and rudimentary in structure and permanently epizoic on the female. In these latter facts we find an analogy in the Copepoda Parasita just noticed. The male has the excretory organ single, median, and probosciform, and placed at the extremity F t <5. 84. 1, ScaJpeJhim rostrattim, Dai-win, Philippine Islands ; 2, Pollicipes cornucopias, Leach, European Seas; 3, Tubicinella trachealis, Shaw, attached to whales ; 4, Acasta sulcata, Lamk., in sponges, New South Wales (4 , tcrgum ; 4 , scutum); 5, Ralanus tintinnabulum, Linn., Atlantic; 5 , Section of Balanus, Linn.; 6, Coroniila diadcma, Linn., attached to whales. of the abdomen, in all these respects differing from other Crustacea in which the male organ is laterally double. In the female organs the ovarian tubes and caeca inosculate together ; there are no oviducts, the ova connected together by membrane, and so forming the " ovigerous lamellae," become exposed by the exuviation of the lining tunic of the carapace or sack, and by the formation of a new tunic on the under side of the lamellae, a process unknown in any other Crustacean. The Thoracica are mainly divided into Balanidce and Lepadidce ; in the former, the animal 1 All the Balanidce are bisexual and hermaphrodite, no males or com- pleinental males having been found in any of them. when adult is inclosed in the parietes of its shell, and fixed to some living or dead object by a broad shelly basis, the aperture being protected by the opercuLir valves. In the Lepadidce the animal is attached by the extremity of a more or less long muscular peduncle, and its body is lodged within the shelly valves of the capitulum. In some species, as in Pollicipes and Scalpellum, the peduncle is covered with more or less numerous rows of scales or squamae. This peduncle in Pollicipes and Scal- pellum corresponds with the basis in Balanus, as may clearly be seen if a Pollicipes with a short peduncle and a Balanus with a deep cup-formed or cylindrical basis be com pared, the animal being in part lodged in both, as in Ibla and Lithotrya. The scales which surround the base of the valves in Pollicipes correspond with the parietes of the walls of Balanus, the valves of the capitulum of the former being homologous with the opercular valves in the latter (fig. 84). The body consists of six, perhaps of seven, posterior thoracic somites. In the division Thoracica the abdomen is undeveloped. 2 The thoracic segments support six pairs of cirri. Each cirrus consists of a two-jointed pedicel, carrying two multi-articulated rami. The mouth has a labrum, palpi, mandibles, and two pairs of maxillae ; within the sack a folded membrane forms the branchite. Darwin concludes that in the Cirripedia the body may be said to be composed, at most, of bat seventeen segments. In order to indicate the homologies which still exist between the parts of an adult Cirripede and an ordinary free 85. Theoretical view of the homologies of the Cirripedia with other Crustacea. A, Leucifer (a Stomapod) ; 15, Lepas. a, antennae; (, e, the eyes ; m, the mouth ; p, the pe-nis. (After Darwin.) Crustacean, we give the accompanying illustrations from Darwin (fig. 85). The upper figure is a Stoma pod Crustacean (Leucifer); the abdomen, being rudi mentary in the adult Cirripedia, is only shown in faint outlines. The lower figure is of a mature Lepas with the antennae and eyes, which are actually present in the larva, retained for the sake of completing the comparison. " All that we externally see of a Cirripede, whether pedunculated or sessile, is the three anterior segments of the head of a Crustacean, with its anterior end permanently cemented to a surface of attachment, and with its posterior end project ing vertically from it" (Darwin). The thoracic appendages of the Cirripedia present us with yet another wonderful modification of the Crustacean type; these biramous multi-articulated cirri are neither natatory, nor ambulatory, nor branchial, but " captorial," or fitted for sweeping the water, and thus catching prey, their alternate extensile and retractile wave-like movements bringing all floating particles and minute organisms within reach of the inclosed mouth. 3 2 In the pupa, however, of this order, and in the mature animal of the two other orders (the Abdominalia and Apoda), it is formed of three segments. , 3 Mr Gosse mentions that the little crab, Porcellana platycheles (tig.
VI. 84Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/699
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