THE ENTBROPNEUSTA. 13 



traction which the body may undergo in preservation. The 

 trouble chiefly occurs in the case of the proboscis-stalk and 

 folds of the collar. By comparing figs. 3 and 4, which were 

 drawn from living specimens in the extended state, with fig. 5, 

 which is taken from a preserved specimen, these relations will 

 be understood. In the contracted animal the gill-slits are 

 always more or less obscure, owing to the great shortening 

 which takes place in the branchial region, together with the 

 protrusion of the valves (fig. 5, vlv.). 



In older animals this contraction is not nearly so great, 

 probably owing to the increased firmness of the branchial 

 skeleton. 



Internal Structure. 



Stages F and G — Skin. — The ectoderm is composed of long 

 fusiform cells arranged two to three deep over the body, except 

 in the dorsal side of the collar groove, where they are colum- 

 nar and one layer thick. Also in the posterior dorsal region 

 the skin is thinner than that of the rest of the body. The 

 whole surface is ciliated. Beyond the fact that the cells are 

 more compressed, and closely arranged, the structure is 

 similar to that of the previous stage. 



Nervous System. — The solid cord which began to separate 

 from the skin in the middle dorsal line at Stage F continues to 

 sink inwards. No lumen is as yet present in it. Throughout 

 its length it still remains in contact with the skin, fusing with 

 it at both ends (figs. 10 and 20—24). 



Towards the end of Stage Gr a differentiation begins between 

 the upper and lower parts of this cord, the upper being formed 

 of cells, while the lower part consists of lightly stained sub- 

 stance, which in older animals is distinctly made up of fibres. 

 Its fibrous nature cannot in this condition be certainly affirmed, 

 probably owing to defect in preservation. The formation of a 

 nervous network over the whole body, which afterwards 

 occurs, is not yet begun. 



Hypoblast. — The mouth is ventrally directed (fig. 7), and 



