MOLLUSCA 



105 



much elongated so as to be vermiform ; mantle-flap as in 

 Neomeniae ; ctenidia in the form of a pair of branchial 

 plumes, one on each side of the anus ; foot aborted, its 

 position being indicated by a longitudinal furrow ; odonto- 

 phore greatly reduced, the radula only represented by a 

 single tooth ; gonads and nephridia as in Neomenia. 



The order Chaetoderma contains the single genus Chx- 

 toderma (fig. 12). 



Further remark* on the Isopleurous Gastropods. The 

 union of the Chitons with the remarkable worm-like forms 

 Neomenia and Chaetoderma was rendered necessary by 

 Hubrecht's discovery (25) in 1881 of a definitely consti- 

 tuted radula and odontophore in his new genus Proneo- 

 menia, founded on two specimens brought from the arctic 

 regions by the Barents Dutch expedition. 



By some writers e.g., Keferstein the Chitons have been 

 too intimately associated with the other Gastropoda, whilst, 

 on the other hand, Gegenbaur seems to have gone a great 

 deal too far in separating them altogether from the other 

 Mollusca as a primary subdivision of that phylum, inas- 

 o much as they are Ulti- 



mately bound to the 

 other Glossophora by 

 the possession of a 

 thoroughly typical 

 and well - developed 

 odontophore. They 

 undoubtedly stand 

 nearer to the archi- 

 Mollusca than any 

 other Glossophora in 

 having retained a com- 

 plete bilateral sym- 

 metry and the primi- 

 tive shell-sac, though 

 the metameric repe- 

 tition of this organ 

 and of the ctenidia is 

 a complication of, and 

 departure from, the 

 primitive character. 

 It is not improbable 

 that in the calcareous 

 spines and plates of 

 the dorsal integument 

 of Neomenia and Chae- 

 toderma, which occur 



FIG. 13.-Diagrams of the alimentary canal of also On the part of 

 Isoplenra (from Hubrecht). o, mouth ; a, the dorSUm Uncovered 

 anus; d, alimentary canal; I, liver (digestive , in- /-xi-, 

 gland). A. Neomenia and Proneomenia. B. by Sfiell in Lniton, We 

 ChKtoderma. C. Chiton. h ave t jj e retention Of 



a condition preceding the development of the solid Mol- 

 luscan shell, or a reversion to it. The minute calcareous 

 bodies may have the same relation to a compact shell which 

 the shagreen denticles of the sharks have to a continuous 

 dermal bone. 



The anatomy of the Gastroj>oda Isopleura has been largely 

 elucidated within the past year by the researches of 

 Hubrecht and of Sedgwick, who have been the first to 

 apply the method of sections to the study of this group. 



The leading points in the modifications of mantle-flap, 

 foot, and ctenidia are set forth in the preceding summaries, 

 and in the accompanying references to the figures. With 

 regard to other organs, we have to note the form of 

 the alimentary canal (fig. 13), which is simplest in 

 Chaetoderma, symmetrically sacculated in Neomenia, and 

 wound upon itself, forming a few coils, in Chiton. The 

 latter has a compact liver with arborescent duct, which is 

 represented by the sacculi in Neomenia and by a single 



caecum in Chaetoderma. Salivary glands are present in 

 Chiton and in Proneomenia. The radula is highly devel- 

 oped in Chiton, and, though present in Proneomenia, has 

 not been described in Neomenia. A single tooth in Chae- 

 toderma appears to represent the radula in a reduced state. 

 The circulatory organs of Chiton alone are known with 

 any degree of detail (fig. 10, C). There is a median dorsal 

 blood-vessel the aorta which is enlarged to form a 

 ventricle in the posterior region of the body. On either 

 side the ventricle is connected to a well-developed auricle, 

 which pours into it the aerated blood from the gills 

 (ctenidia). The extent to which vascular trunks are 

 developed has not been determined, but vessels to and 

 from the ctenidia, and in the mid-line of the foot, are 

 known. As in other Mollusca, the vessels do not extend 

 far, but lead into lacunae between the organs and tissues. 

 Dorsal and ventral vessels have been detected in Neomenia 

 and Chaetoderma, but no specialized heart. 



A B 



FIG. 14. Diagrams of the eicretory and reproductive organs of Isoplenra (after 

 Hubrecht)L 0, ovary ; P, pericardium ; A T , nephridinm ; , external apertnre 



of nephridinm ; y. external apertnre of the genital dnct of Chiton ; r, rectum ; 

 CZ, cloacal or pallia! chamber of Neomenise and Chartoderma ; Br, ctenidia 

 (branchial plumes). A. Chietoderma. B. Neomenia. C. Proneomenia. D. 

 Chiton. 



The heart of Chiton lies in a space which is to be 

 regarded as a specialized part of the ccelom, and, as in 

 other Molluscs, is termed the pericardium. In front of 

 this space in Chiton lies the ovary (fig. 14, D). In the 

 other Isopleura the genital bodies (gonads) lie in the peri- 

 cardium, which has a longer form and extends dorsally 

 above the intestine. Opening into the pericardium equally 

 in all the Isopleura (fig. 14) is a pair of bent tubes which 

 lead to the exterior. These are the nephridia, which in 

 Chiton are essentially renal in function. Their disposition 

 has been determined by Sedgwick (26), who has shown that 

 each nephridium is much bent on itself, so that, as in the 



O 



