OBSERVATIONS ON POLYZOA. 



225 



coextensive with the first, and is the principal membrane of 

 the endocyst. The size of its cells does not vary apprecia- 

 bly within the limits of the group. As a general rule, 

 however, its thickness in the coenoeciurn is quite double 

 what it is in the evaginable endocyst, and on the outer side 

 of the tentacles. On the inner side of those organs, and in 

 the lophophore, it becomes as thick as it was in the coenoe- 

 cium. In the arms, also, it is thicker than in the evagina- 

 ble endocyst; but its greatest development in this respect 

 is attained in the region of the sphincter muscles (Fig. 

 8, E"). This membrane is thicker than the first membrane 

 in the other parts of the coenoecia of Fredericella and Plu- 

 matella, but thinner than the first membrane in the coenoe- 

 cia of Pectinatella and Cristatella (Figs. 7, 8, E"). 



(3) The third layer is exceeding- 

 ly contractile. The transverse fibres 

 of which it is composed are loose- 

 ly and irregularly set, but have con- 

 siderable muscular power (Figs. 4, 

 8, E'"). The coenoecial endocyst 

 of Plumatella is sometimes drawn 

 in by annular constrictions, hap- 

 pening, apparently at will, in any 

 part of the wall where the ectocyst 

 is sufficiently pliable, which are 

 generally referable to the action of 

 this muscular coat. Such annular 

 constrictions can be occasionally 

 traced to rows of small muscles ex- 

 tending across the cavity from the 

 endocyst to the alimentary canal, 

 or to the invaginated fold. These muscles, however, could 

 not have been the cause of the constrictions in the dead 

 Plumatella Arethusa figured in PI. 8, fig. 10. The most 

 careful observations of this specimen, with a high power, 

 failed in bringing to light any such rows of muscles, and, 

 in this case, all the plications, with the exception of those 

 brought about by the influence of the retentor muscles (PI. 

 8, fig. 5), were due to the transverse annular muscles of 

 the third layer. 



An involution of this layer aids in forming the base of 



FIG. 7, base of a coenoecial 

 wall of a living specimen of 

 CRISTATKL.L.A.OPHIDIOIDEA 



viewed from the base, the 

 specimen being inverted : E', 

 first membrane of the endo- 

 cyst : E", second membrane, 

 of the endocyst : E'", third 

 layer of transverse muscular 

 fibre : E"", epithelial mem- 

 brane with longitudinal mus- 

 cular fibres. 



