No. 569] 



REGENERATION 



There are three Polian vesicles., one fusiform and an inch in length, 



Clark ('02) gives the number for Thyone briareus 

 (Leseur) as usually one or two; for T. scabra (Verrill) 

 as usually single, and for T. uniscmita (Stimpson) as 

 one. He also mentions six other holothurians found in 

 the Woods Hole region and all have a single Polian ves- 

 icle except C licit maria frondosa (Gunnerus), which usu- 

 ally has one. He says nothing of the position in which 

 these vesicles are found. In another paper ('01) Clark 

 mentions a large holothurian about 40-45 centimeters in 

 length (Holothuria mexicana Ludwig) in which there is 

 a great diversity in the number of tentacles and Polian 

 vesicles. The tentacles vary from 18 to 21, while the 

 Polian vesicles vary from 1 to 9. The number of speci- 

 mens examined, sixteen, was hardly sufficient to obtain an 

 adequate comparison ; two had 1 vesicle each, two had 2, 

 five had 3, three had 7, one had 8, and one had 9. It is 

 probable that if one were to examine a large number of 

 individuals of each species, with reference to the number 

 and location of the vesicles, he would obtain further inter- 

 esting results. Lang ('96) cites a number of groups of 

 holothurians in which only one vesicle has been observed ; 

 but states that there are a number of species in other 

 groups that have occasionally or usually more than one. 



Where accessory vesicles occur they vary greatly in number, and ap- 



Polian vesicle occurs it lies in the left ventral interradius, very seldom 

 in the left dorsal interradius. Where two or more vesicles occur, they 



Since Lang describes Cucumaria as the type specimen, 

 in which the Polian vesicle is said to be in the left ventral 

 region, it is possible that his generalizations were based 

 principally on this form. At any rate, the conditions in 

 Thyone seem to give a more definite significance to the 

 number and location of the Polian vesicles. 



Various explanations of autotomyand evisceration have 

 been suggested, many of them having a teleological char- 

 acter. The view that the holothurian offers up the better 



