248 J A MI'S KKNKST KINDRED. 



body which can be supplied by free oxygen of the perivisceral 

 fluid. But in the Holothuroidea of the- types like Cucunmria 

 and Thy one, the body is made up of highly contractile muscle 

 elements which may possibly need more oxygen than can readily 

 be supplied by the perivisceral fluid. Consequently, it is sug- 

 gested that the hemocytes, as oxygen carriers, have appeared in 

 association with the development of the muscular system and 

 greater muscular activity, ?o that the contractile elements may 

 be supplied. Further evidence for this suggestion is found in the 

 absence of these hemocytes in the Holothuroidea without mus- 

 cular bodies (e.g., P solus] which are comparable to the test- 

 bodied Echinoidea. 



4. The Vibratile Corpuscles. 



The vibratile corpuscles are those cells in the perivisceral fluid 

 which are minute and bear flagella. Cells of this type were 

 observed in Ophiopholis aculeata (Fig. 42), 5. drobachiensis 

 (Fig. 43), S. franciscanus (Fig. 44) and Stichopus calif ornic us 

 (Fig. 41). 



In 0. aculeata the vibratile corpuscle is very minute (3 microns 

 in diameter). A small peripheral layer of granular cytoplasm 

 encloses a relatively large nucleus. A single long flagellum is 

 present at one end of the cell. The vibratile corpuscles of 

 Strongylocentrotus are much larger, each has a relatively smaller 

 nucleus as compared with the amount of cytoplasm and a shorter 

 flagellum than the vibratile corpuscle of 0. aculeata. 



In S. calif ornicus , the vibratile corpuscles are very minute 

 (i micron in diameter) and are colored with a yellowish pigment 

 which obscures the nucleus, if such be present. Since hemocytes 

 are lacking in this species it may be assumed that these cells are 

 oxygen-carrying cells and that the pigment is a hemin compound 

 related to the content of the hemocytes of the other muscular- 

 bodied Holothuroidea. 



Cuenot ('02) suggested that the function of the vibratile cor- 

 puscles is to keep the fluid content of the perivisceral cavity in 

 motion, thus aiding the ciliated peritoneum in causing a circula- 

 tion of the fluid. 



