THE FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 9 



According to the figures before us, the mesoderm in Chiton shows a specially 

 primitive condition, the coelomic sacs being very large, and by far the greater 

 part of the mesoderm formed 'up to this time being related to them (Fig. 4, A 

 and B}. In other Molluscs they are much smaller and do not assume the 

 characteristic shape found in the Annelida. As a rule, the mesodcrm-bands 

 disintegrate very early. In the remains of these bands, if not also in the 

 scattered portions of mesoderm, a division into layers takes place which gives 

 rise to the pericardial cavity (which is thus coelomic in origin). It would be 

 of great value to ascertain the relation of the pericardium to the first (bilateral 

 and bilaminar) mesoderm-rudiment in the Amphineura, especially as in some 

 of them we find very primitive conditions prevailing in that the genital glands 

 are immediately connected with the pericardium and the genital products thus 

 pass direct into the latter, as, in the Annelida, they pass into the body- 

 cavity, and are conducted thence to the exterior by the nephridia (Soleno- 

 gastres). 



The central nervous system in Chiton consists of the oesophageal 

 ring and four large 

 nerve-trunks, two pedal 

 and two visceral, run- 

 ning from it. The 

 anterior portion of the 

 nervous system is con- 

 stituted by the cerebral 

 ganglia which arise 

 from the ectoderm as 

 two thickenings lying 

 side by side at the 

 anterior part of the 

 body. According to 

 KOWALEVSKY, these 

 ganglia lie at the 

 frontal pole beneath 

 the ciliated tuft (Fig. 

 5, eg), constituting the 

 apical plate (see p. 6, 

 and below). The pedal 

 and visceral commis- 

 sures similarly arise 

 from the ectoderm by 

 the differentiation and 



separation of cell-strands, stages of which may be seen in Fig. 4 

 A -C, pn, In. 



^.s. 



FIG. 5. Longitudinal section through an embryonic 

 Chiton (after KOWALEVSKY). eg, cerebral ganglion ; 

 fd, pedal gland ; kg, visceral ganglion ; m, mouth ; 

 md, enteron ; mes, mesoderm ; oe, stomodaeum ; r, 

 radular sac ; s, shell-rudiment ; w, ciliated ring 

 [velum] ; ws, ciliated tuft. 



