DEYELOPME]!irT OF THE PHYLACTOL^MATOUa POLTZOA. 491 



to have been aware of the researches of Hyatt, made a short 

 time previously ; and though he has, to a certain extent, been 

 anticipated by these, his memoir has all the value of independent 

 investigation. 



Nitsche's observations agree, on the whole, with those of 

 Hyatt, but in some important points supplement them ; and we 

 thus, from the combined labours of the American and Grerman 

 zoologists, have attained to a very accurate and complete know- 

 ledge of the structure of the Phylactolaematous Polyzoa. The 

 following may be regarded as the most important results of 

 these researches. 



Structure of Endocyst. — In the endocyst three distinct layers 

 may be demonstrated. These are (in succession from without 

 inwards) : — 1, an outer cellular layer ; 2, a muscular tunic ; 3, a 

 ciliated epithelium. 



The outer cellular layer is composed of two different forms of 

 cells. The cells composing the principal mass of this layer are 

 prismatic where they enter into the proper body-walls, while in 

 the tentacular sheath or invaginable portion of the body-wall 

 they have become diminished in height and increased in width 

 so as to assume the form of flat polygonal cells. In every case 

 they show a manifest cell-membrane, and possess a large and dis- 

 tinct nucieolated nucleus. Hyatt makes the interesting observa- 

 tion that in the tentacular sheath these cells are eminently con- 

 tractile, occasionally dilating to twice or thrice their normal size, 

 and then suddenly contracting. 



Imbedded among the polygonal cells we find those referable 

 to the second form. These are roundish or oval, also with 

 manifest membrane and with small parietal oval nucleus. Their 

 contents, which in the living animal are clear and strongly 

 refringent, become quickly and intensely coloured by carmine 

 solution, while the polygonal cells are scarcely aff"ected by the 

 colouring-matter. 



In the muscular tunic two distinct sets of fibres may be de- 

 tected, an outer circular or transverse set, and an inner longitu- 

 dinal set, both supported by a common delicate homogeneous 

 foundation membrane. Both circular and longitudinal fibres are 

 smooth, more or less flattened, contain a nucleus, and lie with 

 their pointed ends wedged between each other. 



The third and most internal layer of the endocyst is the 



ciliated epithelium. JSTitsche has noticed that the cilia do not 



36* 



