228 HYATT, OBSERVATIONS ON POLYZOA. 



undoubtedly present, being occasionally seen in a direct 

 view; but, when looked for in a lateral section, they are 

 too diaphanous and closely adherent to the fourth mem- 

 brane to be defined. Their incorporation with this mem- 

 brane will also be justified by the description of its func- 

 tions in connection with the alimentary canal of Pectina- 

 tella. 



er. The endocyst is consequently made up of three membranous and 

 two muscular layers ; all the specialized constricting muscles of the 

 body being derived from the third layer, and all the longitudinal from 

 the fourth layer of muscular fibre. 



The inner and outer tentacular bands, as will be presently shown, 

 are inseparable from the latter layer. The retractors, also, notwith- 

 standing their disc like structure, can hardly be distinguished from the 

 numerous abnormal bands, that occur in some species, connecting the 

 endocyst and alimentary canal. These undoubtedly belong to the 

 fourth layer, and the retractors may, therefore, be looked upon as hav- 

 ing the same relation to the fourth layer that the sphincter has to the 

 third. 



The peculiar arrangement of the third and fourth layers retains the 

 form of the parts, and gives stability to the entire endocyst. By the 

 contraction of the third and relaxation of the fourth the transverse 

 diameters of the parts may be decreased, and the longitudinal increas- 

 ed; or. by the opposite process, the longitudinal may be decreased, and 

 the transverse increased. During the invagination of the potypide, the 

 fibres of both are in a state of contraction in the evaginable endocyst 

 and in the region of the sphincter; in the ccencecium, however, the}' 

 are relaxed. But as soon as evagination begins, they appear to reverse 

 this condition. The caeuaecial fibres become contracted and those of 

 the same layers in the potypide are stretched to their full length. By 

 these reciprocal changes they materially assist the compressed fluids of 

 the body in forcing out and expanding the polypide. I have, also, had 

 reason to doubt the existence of a neural diaphragm. In examining a 

 specimen of Fredericella regina from the side and from above, under 

 very favorable circumstances, I was unable to detect the same appear- 

 ance of an enveloping membrane just below the nerve mass, that led 

 me to the conclusion mentioned ; nor have I had any opportunity of 

 verifying my first observations on Pectinatella, which, however, were 

 faithfully made with one of Tolle's one half inch objectives. Until 

 therefore, Fredericella is shown to be exceptional in this respect by 

 further observations on other genera, it is, perhaps, best to regard 

 the existence of a neural diaphragm as doubtful. 



[To BE COTIXCED IX VOL. V.] 



