222 HYATT, 



genera ; and the absolute number of the buds to the expan- 

 sion of the bud producing surface. As has been shown in 

 the preceding paragraph, the number of the buds reaching 

 maturity, and their absolute number in each cell of Frederi- 

 cella, is generally less than in those varieties of Plumatella 

 that have a gelatinous ectocyst ; and they are less, in the 

 latter, than in the Pectinatellap, which have no ectocyst; 

 and less in the Pectinatellas than in Cristatella, where the 

 ectocyst is wanting, and where the bud producing surface 

 is of the greatest extent. 



COMPOSITION OF THE ENDOCYST. 



In the foregoing remarks the anatomy has been discuss- 

 ed, so far as was necessary, in order to give clearness to 

 the descriptions of the different genera and the subsequent 

 notice of the modes of reproduction. It now remains to 

 consider more fully the composition of the body, together 

 with the relations and functions of the various organs. 



The endocyst is made up of four layers: (1) an outer 

 large celled membrane (PL 11, fig. 1, E', PI. 12, fig. 2, E', 

 PL 13, fig. 16) ; (2) an inner one of smaller cells (PL 11, 

 fig. 1, E"); (3) one of muscular fibre (PL 11, fig. 1, E'") ; 

 (4) an epithelial layer lined internally with muscular fibre 

 (PL 11, fig. 1, E""). 



(1) The first membrane forms the external surface of 

 the endocyst of the polypide and of the coenoecium. 



The cells on the ccencecia of Fredericella and Plurnatel- 

 la are hexagonal containing a large brilliant nucleus and 

 nucleolus (Figs. 3, 5). Their upper sides are depressed 

 by the weight of the superincumbent ectocyst, and their 

 longitudinal diameters are not so long as in the cells of the 

 same membrane on the evaginable endocyst. 



When fully expanded on the living coenoecium the cells 

 are closely pressed one against another ; but, if treated with 

 alcohol, they contract, and, separating from each other, leave 

 wide intervening spaces (Fgs. 2, 4, 5). These spaces have 

 been figured by Prof. Allman under the impression that they 

 were anastomosing channels, perhaps blood channels ; my 

 observations, however, have been too numerous to leave any 



