THE DEVELOPMENT OF THE LARVAL FORM. 5 



time longer continues to deepen (Fig. 2 A, oe}. This ectodermal 

 invagination, the stomodaeum, represents the rudiment of the fore-gut 

 (buccal mass and oesophagus). In connection with it there appears 

 later, as a ventral outgrowth, the radular sac (Fig. 2 B, r). 



The " shifting " of the blastopore just described agrees closely with the 

 processes to be met with in the Gastropoda (p. 141), and we are inclined in both 

 cases to assume that we are really dealing with the closing from behind forward 

 of an originally slit-like blastopore. 



a. js 



Flo. 2. A and B, median longitudinal sections through embryos of Chiton Polii at 

 different ages (after KOWALEVSKY). fd, pedal gland ; m, mouth ; md, enteron ; mes, 

 mesoderm ; oe, stomodaeum ; r, radular sac ; w, ciliated ring (velum). 



The more active growth of the part lying behind the ciliated. ring 

 is accompanied by reduction of the anterior section which formerly 

 preponderated (Figs. 1 and 2). The embryo, which is now almost 

 pear-shaped, may become free at this stage (e.g., Chiton marginatus, 

 LOVEN). The larvae of this latter form carry a large ciliated tuft at 

 the frontal pole (Fig. 3 A). The embryos of other Chitones remain 

 longer in the egg, and before attaining free life approach more nearly 

 the form of the adult (Fig. 3 (7). 



The larvae of the Chitones resemble those of the Annelida, and since a 

 TrocJwpJiore exceedingly like that of the Annelida is found in other Molluscs 

 (Figs. 18, 51, 53), we are justified in instituting such a comparison here also, 

 even although the resemblance is not so close. We have here a pre-oral 

 ciliated ring, and the origin and position of the different sections of the 

 intestinal canal is the same as in the Troclwplwre. The larva, at first, has no 

 anus, as the terminal segment of the alimentary canal only appears later at 

 the posterior end of the body in the form of an ectodermal invagination, the 



