234 J. H. OKTON. 



It is unfortunate that Diiben and Koren did not figure the calcareous 

 collar and polian vesicle of the type specimens, for Barrois has described 

 a form which only differs from T. raphanus in having a bifurcated polian 

 vesicle. Neither of these authors, however, found more than one speci- 

 men. The Plymouth specimens agree with Barrois' form except in the 

 calcareous collar as figured by Barrois and the presence of tube-feet on 

 the "tail" (cf. 6, Plate I, with Fig. 11, p. 231, and Fig. 13, p. 233), and 

 they agree also with Marenzeller's form except in the polian vesicle 

 as figured by Marenzeller. As, however, Theel (19) had already doubted 

 the distinctness of T. foucheti, it is very probable that all these form.s 

 belong to Diiben and Koren's species T. raphanus. There is some ground 

 for believing that Barrois' figure of the delicate calcareous collar of his 

 Thyone is not a typical one ; and Marenzeller's figure of a bifurcated 

 polian vesicle requires confirming in more specimens. Therefore for 

 the present it has been deemed advisable to refer the Plymouth forms 

 to T. raphanus Diib. and Kor. If, however, the type of the latter 

 species is found to possess constantly a bifurcated polian vesicle, then 

 the forms from this district probably belong to T. poucheti Barrois. 



Several individuals of this species were kept under observation alive 

 for some months in the same dish with the C. elongata mentioned above. 

 These specimens maintained their tails above the surface of the sand, 

 as is shown in Fig. 11, in the same way as did the Cucumarian. The 

 tentacles have, however, only been seen expanded above the surface of 

 the sand on two occasions when the water was becoming foul, and it 

 was observed that the body was maintained bent in a U-shaped manner. 

 The tentacles in this species are very short (see Fig. 12), as in C. elongata. 

 This reduction of the tentacles and buccal retractors and the correlated 

 tailed posterior end are also found in the group Malpodiida, which also 

 inhabit muddy situations and have been found to ingest the muddy sand 

 in which they live (15). Doubtless these characters of the tentacles are 

 related to the manner in which all these animals feed, for short stiff 

 tentacles would be much more useful for ingesting mud or fine sand than 

 the long dendritic tentacles such as are found, for example, in some other 

 species of Cucumaria and Thyone. Thus these mud-dwelling Holothurians 

 form one more example of that interesting phenomenon in nature, 

 namely, the occurrence of similar adaptations in different animals for 

 performing the similar functions necessitated by a similar mode of life. 



