GENERAL CONSIDERATIONS ON" THE MOLLUSCA. 317 



likewise to be the rase in the Chitons which in other respects also 

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

 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 adults, show the secondary body cavit \ 

 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 even 

 quite small. The primary body-cavity with the mesenchymatous 

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

 cavity, 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 ► v them, though only slightly developed. 



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

 usually completely disintegrate or at the most persist to a small 

 sxtent 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 oi'gans 

 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). 

 There run consequently no longer be any doubt that Hi" pericardium of 

 the Mollusca should I" regarded as the secondary 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 



