ZOOL.— Vol.. II.] ROBERTSON— ENTOPEOCTA. 333 



occurred. In older polypides, the separation took place just 

 as often between stalk and stolon as between calyx and 

 stalk. In younger individuals, out of fifteen experimented 

 with, twelve broke either at the junction of stalk and stolon, 

 or else part way up the stalk, and only three broke at the 

 septum between stalk and calyx. Experimenting with other 

 Pedicellines, the writer could in no case detect any stretch- 

 ing of the stalk. The calyx always separated at the septum 

 and with the very slightest pressure. In the colonies of 

 Myosoma which have been examined there is a remarkably 

 small number of headless stems. It was only after a care- 

 ful search among stems accidently broken from stolons 

 while tearing them out that a few were obtained upon 

 which calyces were forming in various stages of develop- 

 ment; so that, although Myosoma does not lose its calyces 

 as easily as most Entoprocta, yet, if the loss occurs, regen- 

 eration does take place. 



C. Discussion. 



Myosoma sfinosa is an extremely interesting form, both 

 as an example of correlated and adaptive variation, and as 

 a form which while exhibiting a high degree of specializa- 

 tion, yet shows marked affinities with more primitive Ento- 

 procts, especially with L.oxosoma. 



The stem of all Entoprocts is characteristically muscular. 

 The calyx, except for delicate sphincter muscles, is just 

 as characteristically devoid of muscles. Two observers, 

 Allman (1856) for Pedicellina echinata, and Van Beneden 

 (1845) for Pedicellina belgica, speak of finding retractor 

 muscles in the calyx of these species. Their results on this 

 point have not been confirmed by other observers, but, even 

 if correct, the muscles to which they refer have an entirely 

 different function from the somatic ventral muscle of Myo- 

 soma. The function of a retractor muscle in the sense 

 spoken of by these observers is to draw the tentacles within 

 the sheath. Indeed, Allman speaks of a structure which 

 he calls a sheath in P. echinata, into which the tentacles are 

 retracted. The ventral muscle of Myosoma, however, does 



