EMBRYOLOGY OF ECHINODERMS. 113 



oral lobe, have lengthened so much that their tips {/), are 

 visible in a ventral view. They are to become the spicules 

 of the pre-oral arms. The fourth branch (^), is now doable, 

 and forms a fork, which runs nearly to the tip of each post- 

 oral arm. 



The digestive tract is now quite complicated. The 

 mouth (m), which has been fonned by the union of the 

 integument to the wall of the digestive tract, is situated 

 in the depression between the ciliated ridge and the oral 

 lobe (a). It communicates through a short oesophagus (o e), 

 with the large, flask-shaped, thick-walled stomach (s). 

 Tlie anus (o), is now very small, and it no longer opens 

 directly into the stomach, but is joined to it by a smaller 

 tube, the intestine (2), which is seen in a ventral view be- 

 tween the body wall and the stomach. 



e. In from twenty-four to forty-eight hours more the 

 larva will be found to have changed greatly, and it is now 

 sufficiently transparent to allow the internal structure to 

 be studied more easily. It is shown in Fig. 66, as it ap- 

 pears in a side view while swimming, and in Fig. 65 it is 

 shown in a dorsal view. The specimen shown in this 

 figure was a little flattened by the pressure of the cover 

 glass which was used to confine it. 



The post-oral arms (c, c), have grown so much that they 

 now make about half the total length of the body, and 

 the two spicules ((7), which form the skeleton, have united 

 to each other at intervals so as to form a ladder-like 

 structure, with two long sides, and a number of cross- 

 bars. The pigment spots are now very large and con- 

 spicuous, and there is a longitudinal row of them along' 

 each arm. 



The outer angles of the oral lobe («), are fashioned into 

 a pair of ear-like processes (a' a'), the rudiments of the 



