COMPARATIVE ANATOMY 



CHAP. 



The number of 



Fig. S4.— Posterior 

 end of the tody of 

 Cheetoderma (dia- 

 gram after HubrecM). 

 1, Gouad ; 2, pericar- 

 dium ; 3, rectum ; 4, 

 nepliridimn ; 5, anus ; 

 6, ctenidium ; 7, 

 cloaca. 



ctenidia in each row varies very mucli in the diflferent species of 

 ChitonidcB ; it ranges from 14 to 75. The row extends along the 

 whole length of the branchial furrow (Fig. 83 A), or else (in 

 Chiton Icevis, 0. Pallasii, and Ghitmiellus) is confined to its 

 posterior half (B, C). 



Solenogastres. — [Proneomcnia, Neomenia, C'Juctoderma). 

 The mantle cavity, in these forms, is much reduced, consisting 

 only of the groove on each side of the rudimentary foot ; it 

 opens into the cloacal cavity, or rather widens to form that 

 cavity. The cloaca is thus the posterior portion of the mantle 

 cavity. In Chcctoderma (Fig. 84) the foot has disappeared, and 

 the mantle cavity is reduced to the cloaca, in which one typical 

 gill lies on 'each side of the anus. These gills are regarded as 

 the last ctenidia of the rows found in the Ohitonidce, which in 

 Ohitondlus and some species of Chiton are already confined to 

 the posterior half of the body. In Neomenia, there is no longer 

 a pair of ctenidia, but a mere tuft of filaments rising from the 

 wall of the cloacal cavity, and in Proneomenia, there are only 

 irregular folds of the cloacal wall. 



On the relation of the gills in the Chitonidce to certain 

 pjatches of epithelium, which may perhaps be considered as 

 osphradia, see the section on Olfactory Organs, p. 165. 



B. Gastropoda. 



The Fisstirellidce (Fig. 85, A and B) among the Prosohranchia stand nearest to 

 the racial form of the Gastropoda. The mantle cavity is anteriorly placed ; into it 

 from behind and above project two long gills feathered on each side ; these lie 

 symmetrically to the middle line, and to the right and left of the anus. The 

 posterior portion of their axes is connected by a band with the floor of the respiratory 

 cavity, while the anterior pointed portion projects freely. 



The fact that in the FissurelUdcc (and related forms) the gills are paired and 

 symmetrical is very significant. It points to the primitive character of these forms, 

 and enables us to compare their gills with those of the lower Lamellihranchia, i.e. 

 the Protobranohia, and of the Cephalopoda. We must, however, again emphasise 

 the generally-assumed fact that the left gill of Fissurella answers to the right gill of 

 the Lamellibranchia and Cephalopoda, and the right gill of the former to the left of 

 the latter, these latter having retained their primitive symmetry in this respect. 

 This assumption becomes the more plausible when we consider that the mantle 

 cavity with its organs originally lay posteriorly on the body, and shifted forward 

 secondarily along its right side. 



The Saliotidce are closely connected with the Fissurellida;. Their spacious mantle 

 cavity is, however, forced to the left side by the great development of the columellar 

 muscle. There are two gills, feathered on both sides, of which the right is the 

 smaller. The axis of each gill has united, for nearly its whole length, with the 

 inner wall of the mantle, and only its anterior end is free ; its tip even projects a 

 short distance beyond the respiratory cavity. 



Although the FissurelUdce and Haliotidce still possess two gills, other Diotocardia 

 have retained only the left (ur) and larger gill of Haliotis. This gill is, however, 

 still feathered on both sides, although this characteristic is obscured in a peculiar 

 manner. The septum or axis of the gill, to the broader surfaces of which the branchial 

 leaflets are attached, and one edge of which had, in Haliotis, already fused with the 



