DEVELOPMENT OP THE PHTLACTOLiEMATOTJS POLTZOA. 495 



a pair of large muscles, one in each arm. These run from the 

 oral region to the extremities of the arms, and serve to elevate 

 their tips. 



The two groups of muscles hitherto known as the " great re- 

 tractors of the polypide " and the " rotators of the lophophore " 

 are brought together by Nitsche under the designation of the 

 " great motor muscles of the polypide." He so names them from 

 a belief that when the polypide is completely retracted its extru- 

 sion may be initiated by the action of these muscles, though when 

 it is only partially retracted the contraction of the body- wall may, 

 by its pressure on the contained fluid, be of itself sufficient to bring 

 about the evagination of the tentacular sheath and the protrusion 

 of the polypide. 



Nitsche farther shows that the individual fibres of these muscles 

 are each enveloped in a distinct sarcolemma, and are provided 

 with a nucleolated nucleus, which lies between the proper muscle- 

 substance and the sarcolemma. In quite young buds the muscle- 

 fibres are found to be as yet short fusiform cells with parietal 

 nucleus. Nitsche has never been able to see a true striation in 

 the muscular fibre, but only a slight transverse wrinkling of the 

 sarcolemma; while the breaking up of the fibrillse into disks, which 

 may be occasionally witnessed, takes place so irregularly, that he 

 cannot regard it as indicating a normal structure. 



The posterior parieto-vaginal muscles are described by Nitsche 

 as continuous with the longitudinal fibres of the endocyst. They 

 are not, like the other special longitudinal muscular bundles (such 

 as the great motors of the polypide), simple structures consisting 

 of a single histological element, but are composed (1) of a foun- 

 dation-membrane formed by a prolongation of the homoge- 

 neous membrane of the tunica muscularis of the body-wall, (2) 

 of muscular fibres which pass inwards in bundles from the longi- 

 tudinal fibres of the body-wall, (3) of an epithelium by which each 

 parieto-vaginal band is enveloped. The muscular fibres of these 

 bands pass upwards on the tentacular sheath, and form its fine 

 longitudinal musculature. 



With this composite condition Mtsche contrasts the simple 

 structure of the anterior parieto-vaginal muscles. These, more- 

 over, are not, like the posterior, continued into the muscular 

 layer of the body-wall. Each fibril of the anterior set is known 

 to present a small swelling, which he has proved by treatment 

 with chromic acid to be a true nucleus. He has also observed 



