EMBRYOLOGY OF ECHINODERMS. 



109 



and forms the dorsal surface, while the two short sides, 

 a, c, and c, b, together make up the ventral surface. 



The orifice of invagination (o), is now situated between 

 the angle (c) , and the posterior end, and the primative di- 

 gestive cavity is no longer in the centre of the body, but 

 bends towards the anterior end. Owing to the opacity of 

 the embryo of Arbacia at this stage, the internal structure 

 cannot be very clearly made out, but careful examination 

 will show that the endoderm and the ectoderm of the 

 anterior end of the body are still quite thick, while the 

 ectoderm is quite thin at the posterior end. In the more 

 transparent embryo of Strongylocentrotus at the same 

 stage, the inner end of the digestive tract may be seen to 

 be constricted off as a 

 small sac, the water pouch; & 



and the mesoderm cells 

 may be made out as an 

 internal layer of cells, lin- 

 ing the body cavity on the 

 inner end of the digestive 

 tract. 



FIG. 60. Ventral view of 

 the same larva. (Drawn from 

 nature by W. K. Brooks. ) 



a. Oral or anterior end. 

 6. Posterior end. c. Ciliated 

 ridge, d. Calcareous spicules. 

 e. Orifice of invagination. 



In a ventral view of the same larva of Arbacia at this 

 stage (Fig. 60), the angle c, (Fig. 59) which is seen in a 

 side view, is found to be the profile of an elevated ridge 

 (Fig. 60, c), which runs across the ventral surface near 

 the anterior end, and divides the body into a large pos- 

 terior lobe (6), and a much smaller anterior lobe (a). The 



