GENERAL CONSIDERATIONS ON THE MOLLUSCA. 317 



likewise to be the case in the Chitons which in other respects also, 

 appear to be very primitive animals (Fig. 4 A and B, p. 8); they 

 do not, however, show any suggestion of the segmentation of the 

 mesoderm (formation of the primitive segments) so characteristic of 

 the segmented animals. 



The Amphineura, even as adnlts, show the secondary body-cavity 

 well preserved. It is still very large, and contains the principal 

 organs, such as the alimentary canal, the liver and the heart. In the 

 higher Mollusca (Lamellibranchia and Gastropoda) it is much more 

 reduced in comparison with the primary body-cavity and is evem 

 quite small. The primary body-cavity with the mesenchymatous; 

 tissue distributed in it is very large and becomes the definitive body- 

 '""////, just as in the Arthropoda (Vol. iii., p. 423). In spite of this, 

 the Molluscs have been regarded as typical Schizocoelata, i.e., as forms 

 devoid of a true coelom. Such a true coelom, however, is retained 1 

 by them, though only slightly developed. 



While in the Arthropoda, the coelomic sacs (primitive segments) 1 

 .usually completely disintegrate or at the most persist to a small 

 extent in the genital glands (Peripatus, Myriopoda) the coelom of the 

 Mollusca is always retained in the form of the pericardium from which 

 the nephridia and the genital glands are formed (Gastropoda) in a 

 manner recalling the primitive conditions in the Annelida. Where 

 the connection between the kidneys, the genital organs and the 

 pericardium has not yet been made evident in the embryo in conse- 

 quence of insufficient investigation, or else where, in consequence of 

 secondary modifications in the forms examined, it can no longer be 

 demonstrated, the anatomical condition of the systems of organs, 

 clearly proves such a connection. In various Molluscs, the cavity of 

 the genital glands is directly connected with the pericardial cavity 

 ( Amphineura, Cephalopoda), the nephridia entering the latter through 

 an open funnel, a condition which recalls the open connection existing 

 between the nephridia of the Annelida and the secondary body- 

 cavity (Amphineura, Lamellibranchia, Gastropoda, Cephalopoda).. 

 T/n'.rf can <-<>ii*e<]uently no longer be any doubt that the pericardium of 

 tin- Mollw'i *hnnld In' /vy/^/v/W ax tlif won.dni-y body-cavity ; and,, 

 apart from the absence of segmentation, the resemblance in this point 

 to the Annelida is very great. 



The condition of the mesoderm and the structures belonging to 

 it is thus evidently of great importance in interpreting the Mollusca 

 it has therefore been considered in connection with the early onto- 

 genetic processes. Hardly less important, however, is the larval 



