284 COMPARATIVE ANATOMY. 



ring or velum, the double row of cells in the latter being very clear. 



(f) Transverse section of an older stage (H). The mesoderm 

 cells have increased in number, and are arranged in two groups at 

 the sides of the stomodsum, between the ectoderm and the 

 endoderm. 



(g) At a later stage — a longitudinal section of which is given in 

 Fig. 2 2, E — the principal feature is a stronger development of the 

 mesoderm, in which a space, the body-cavity, now appears. A 

 bulging backward of the stomodaeum forms the first rudiment of 

 the radular sac. Behind the mouth, a sac-like depression is 

 formed, evidently by the ectoderm ; this has been called the pedal 

 gland, although it has not as yet been discovered what becomes of 

 it in the adult animal. 



(h) When the body-cavity forms, the cells of the mesoderm 

 become divided into two layers, the inner visceral layer becoming 

 appended to the intestine, and the outer parietal layer to the ecto- 

 derm {cf. Fig. 2 2, /). In the transverse section, we see, deep 

 down in the ectoderm, the first rudiments of the pleurovisceral 

 cords. The pedal cords arise in the same way, and anteriorly, in 

 the cephalic area, which is encircled by the preoral ciliated ring, 

 the rudiments of the supra-a;sophageal central nervous system form 

 a neural plate — i.e., as a thickening of the ectoderm, which carries 

 a tuft of long cilia. 



(i) At later stages (F.K.L.) the central nervous system, with 

 the pleurovisceral and pedal cords, become detached from the 

 ectoderm and take up their mesodermal position. The rudiments 

 of seven shell-plates appear on the back as cuticular formations ; 

 the eighth only appears later. A posterior invagination of the 

 ectoderm represents the rudiments of the proctodeum (the embry- 

 onic hind-gut, with the anus). The first teeth appear in the 

 radular sac. The whole of the cephalic area and the region of the 

 foot become covered with cilia. On the dorsal ectoderm, on the 

 parts that are not covered by the shell-plates, the first calcareous 

 spines appear. In the posterior part of the body; a great accumu- 

 lation of mesodermal elements evidently marks the position of a 

 formative mesodermal zone. 



At this stage, the larva leaves the egg envelope and swims 

 about freely, and, on the degeneration of the ciliated ring, sinks to 



