226 



HYATT, 



the walls in the coenoecium of Cristatella (Fig. 7, E'"), but 

 it does not probably extend into their reticulated portion 

 (PL 14, fig. 1, Q,). Judging from the thinness of the lat- 

 ter, and, from the fact, that all the longitudinal muscles of 

 the body appear to be connected more or less with the 

 fourth or epithelial membrane, it is quite likely that the 

 reticulated portion, or those parts of the coencecial walls 

 which lie between the junctions of the walls with the up- 

 per and lower internal surfaces of the coenoecial endocyst, 

 are composed wholly of longitudinal fibres, encased by the 

 epithelium. Around the invaginated fold of Plumateila 

 the fibres are thickly disposed and form the so called sphinc- 

 ter muscle. This is not a narrow band, as described by 

 Prof. Allman, encircling the lower edge of the fold, but a 

 local development of the transverse fibres, as broad as 

 the fold itself (Fig. 8, E'"). 



In the evaginable 

 endocyst this layer 

 can be seen, by 

 careful focussing, 

 through the fourth 

 membrane, but only 

 with ease, when 

 more or less contrac- 

 ted. 



In the arms and 

 lophophore it is 

 thick, and frequent- 

 ly, in the former, be- 

 comes convoluted 

 by the action of muscular bands apparently developed in 

 its substance. I was unable to trace this membrane in 

 the tentacles, but judging from the great thickness of the 

 second tentacular membrane, and the slight increase which 

 takes place in the transverse diameter of those organs when 



Fig. 8, Magnified view of the Invaginated fold 

 of a living specimen of Pli'.matella diffusa.* 

 A"", cuenoecial orifice : D, eetocyst : E', first mem- 

 brane of the endocyst : E'', second membrane of 

 the endocyst : E'", third layer of transverse mus- 

 cular fibre, constituting the Sphincter muscle. 

 E"", fourth or epithelial layer, accompanied by 

 longitudinal muscular fibres " 



*Xote. Only three membranes are delineated in the coenoecial endo- 

 cyst of figure 8. This is owing to my want of success in defining the 

 parts of the innermost layer, in the specimen figured, which is undoubt- 

 edly made up of two layers, as in the ccenoecium of Cristatella (Fig. 7, 



