164 BULLETIN OF THE 



The numerous dermal canals close under the hypodermis of S. nudus 

 are unquestionably of great value in respiration, and the region of the 

 introvert, which is distinguished by thin cuticular and muscular layers, 

 actually not so thick as the walls of the tentacular fold, presents a far 

 greater surface for the transmission of oxygen directly to the coslomic 

 fluid than the entire vascular system. 



Primarily, then, this system is hydrostatic, and this is probably its 

 chief function in S. nudus. The dorsal and ventral vessels are reser- 

 voirs into which the fluid is driven by the contraction of the tentacular 

 fold. Ou the other hand, the muscular walls of these vessels serve to 

 furce the fluid out into the lacunae of the tentacular fold, and thus to 

 move and expand the latter. The varying contraction of these two sets 

 of muscular elements gives rise to the constantly changing form of the 

 tentacles, as the fluid is driven to and fro. This movement might easily 

 simulate, or even under certain conditions become, a circulation. More- 

 over, any method of killing which worked violent contraction would dis- 

 tort the tentacular fold by driving the fluid into the extreme distal ends 

 of the lacunae, or by drawing together the entire mass of the tentacular 

 fold, and forcing the fluid back into the dorsal or ventral vessel. It is 

 probably in this way that the lobed or cut form was produced, which 

 has been given as the typical one in all generic descriptions hitherto 

 published. It is worthy of notice that those animals which were killed 

 with expanded tentacles showed the walls of both dorsal and ventral 

 vessels almost in contact, whereas in those which had retracted their 

 tentacles these vessels were so filled by masses of coagulum as to reach 

 a considerable diameter. The probable function, then, of the dorsal and 

 ventral vessels is to receive and hold the fluid forced out of the tenta- 

 cles at the time of inversion of the introvert. 



e. Aboral Wall. 



The aboral wall of the tentacular fold differs from the oral chiefly in 

 the undifferentiated condition of the hypoderm. The latter is here com- 

 posed of low non-ciliated cells, identical with the hypodermis of the 

 general body wall, except where it is elevated into the papillae or sensory 

 organs already described. Sensory cells are wanting. The cutis of the 

 aboral wall lacks the vacuoles which characterize that of the oral wall, 

 and there are only very few leucocytes to be found. 



The thin cuticula, cilia, and sensory cells of the oral, as well as the 

 <reneral sense organs of the aboral, wall of the tentacular fold, show it to 

 be a most important organ of touch. This view is strengthened by the 



