THE DEVELOPMENT OP APLYSIA PUNCTATA. 523 



to the right and left liver aud to the kidneys, and later give 

 rise to the muscles. When the ectoderm cells of the secondary 

 kidney have just sunk below the surface, and before they 

 have become grouped together to form a definite organ, an 

 irregular aggregation of mesoderm cells appears in this 

 region, just anterior to the anal cells (Text-fig. 16). In Text- 

 fig. 17, slightly latei', these cells have formed a definite little 

 mass close beside the now clearly developed secondary kidney, 

 and in the next stage (Text-fig. 18) they are seen to bound a 

 narrow slit-like cavity. This is the coelom. In Text-fig. 19 

 it has begun to extend anteriorly and dorsally so as to cover 

 the dorsal wall of the secondary kidney and the right and 

 antero-dorsal surface of the right liver. In the last stage, 

 before the emergence of the embryo from the capsule (PI. 22, 

 fig. 7), the coelom forms two lateral sacs (coloured red in the 

 figure), that on the right being the larger, connected with one 

 another by two transverse passages — one lying in front of, 

 and the other behind, the right liver, which thus projects 

 dorsally between them. In the free-swimming larva (PI. 22, 

 figs. 8 and 9) this subdivision of the coelom has disappeared 

 by the union of the anterior and posterior passages on the 

 dorsal surface of the right liver. The body-cavity now con- 

 sists of a considerable thin-walled sac, lying on the dorsal 

 side of the larva aud covering the stomach, intestine, the 

 right, and a great part of the left liver, and the posterior half 

 of the secondary kidney. Its ventral extension, however, is 

 nowhere very great. 



Text-figs. 11-14 show the development of the coelom 

 in diagrammatic transverse sections through the region of the 

 right liver and the secondary kidney. 



Hitherto the existence of the coelom in Opisthobranch 

 larv^ has passed unnoticed. Mazzarelli, it is true, mentions 

 a pericardium, which he describes and figures as a small oval 

 sac, but in Aplysia, as we have shown, the coelom is of con- 

 siderable extent and irregular shape. It would seem that 

 Mazzarelli only observed the coelom in whole preparations, 

 which would account for his describing it as a small sac, for 



