Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/670

Rh 642 much elongated so as to be vermiform ; mantle-flap as in Neomenice ; 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 Clix- todcrma (fig. 12). Further remarks on the Isopleurous Gastropods. The union of the Chitons with the remarkable worm-like forms Neomenia and Chaetoderma was rendered necessary by Ilubrecht 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 much as they are inti mately bound to the other Glossophora by the possession of a thoroughly typical and Avell - 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 andChaj- toderma, which occur Fio. 13. Diagrams of the alimentary canal of a l so Oil the part of Isopleura (from Hubrecht). o, mouth ; n, the doi SUlB Uncovered anus ; d, alimentary canal ; 7, liver (digestive . in- /-NI, gland). A. Neomenia and Proneomenia. 15. by shell 111 Chiton, W6 Cluetodenna. C. Chiton. } lave t]le rete ntion 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 Gastropoda Isopleura has been largely elucidated within the past year by the researches of Jlubrecht 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. &quot;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 [iSOPLEUROUS GASTROPODS. 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 Cha&amp;gt; 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 gill,^ (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. C7, cloacal or pallial chamber of Neomenup and Chretoderma ; Br, ctenidia (branchial plumes). A. Chtetoderma. B. Neomenia. C. Proneomenia. D. Cb Chiton. The heart of Chiton lies in a space which is to be regarded as a specialized part of the coelom, 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