376 RIOHAED EVANS. 



(a) The Flagellated Layer. — In contrast with what 

 occurs in the greater number of sponge larvae, the flagellated 

 layer completely surrounds the inner mass^ each of its consti- 

 tuent cells being provided with a flagellum (figs. 38 a, b, c, and 

 d). All the flagellated cells present the same characters, 

 only differing slightly from one another in length ; and the 

 nuclei, which in all cases are situated near the bases of the cells, 

 are consequently not on the same level (figs. \a, 5, 6, 7, and 

 38 a and b) . Their general form is, on the whole, a charac- 

 teristic one so long as the flagellated layer retains its position 

 at the surface. The cell body, which lies almost altogether 

 externally to the nucleus, is constricted so as to present the 

 appearance of having a waist. The narrower part measures 

 no more than 1| /i across; while the broader part, which is 

 situated externally, measures from 2 to 2| fi, the length of the 

 cell varying from 5^ to 7\ p. 



Owing to the constricted condition of the middle portion of 

 the cells, spaces are often seen between them. The nuclei, on the 

 other hand, are closely wedged against one another, while the 

 external ends fuse with one another to form what might almost 

 be described as a thin membrane in which no cell outlines can 

 be distinguished. The cytoplasm contained in the external 

 position is more opaque than that existing in the waist of the 

 cell. This condition is probably due to the presence of very 

 fine granules, which give it a denser consistency. The cells 

 appear as if they were suspended from this membrane, only 

 touching one another at their inner ends where the bulging 

 nucleus is situated (figs. 1 and 5). 



The flagellum with which each cell is provided lies in part 

 outside and in part inside the cell. The portion which lies 

 outside is at least as long as the cell itself, and appears to 

 taper to a point, its base being in some cases surrounded by a 

 cone-like elevation of the cytoplasm. The portion which lies 

 inside the cell passes down to the neighbourhood of the nucleus, 

 and in many cases presents a small swelling which lies on 

 the nuclear membrane, and possibly represents the centro- 

 some. Sometimes the flagellum, instead of ending in a small 



