SPONGES 



55 



collar cells, Savile-Keiit, and others), but in more recent times they have 

 not been seen by any observer, and their existence must be considered 

 doubtful. 



The cytoplasmic reticulum is clear and as a rule not very granular, 

 but usually contains one or more coarse refringent granules, similar to 

 those found in the dermal epithelium. There are commonly found also 

 a few irregular granulations, perhaps food particles. The nucleus is 

 rounded, slightly irregular in form, and always attached in Clathrinidae to 

 the surface of the cell. It contains usually a distinct nucleolus, and an 

 irregular, blotchy, nuclear reticulum. 



The single flagellum is long, slender, and of even thickness throughout 

 its length. It arises in Clathrinidae always from 

 a distinct granule of peculiar staining properties, 

 situated at the summit of the cell. When the cell 

 is fully expanded, the flagellum in preparation 

 appears homogeneous and difficult to see ; but in 

 the contracted state it is dark, granular, and ap- 

 parently very brittle (Fig. 52, B, d). During life 



Fio. 53. 



Choanoflagellata, after France. On the left, Codonosiga botrytis, J. Cl., X350, showing the 

 commencing transverse fission. In the middle, Salpingoeca fusiformis, S. K., X500. On the 

 right, Diiilnxlijn Ent-H, France, x400. col, collar ; i.col, e.col, in Diplosiya, internal and external 

 collar ; fl, flagellum ; st, stalk ; th, theca. 



it appears, in side view, to have a rhythmical stroke from side to side, 

 with a longer pause on one side than on the other ; the beats in the 

 collar cells of Sycon are normally about ten to the second (Bidder). 

 Seen in surface view the flagella show a whirling movement, each one 

 moving quite independently of its neighbours (Vosmaer and Pekelharing 

 [30]). 



The collar, the most characteristic feature of the cell, is in Ascandra 

 falcata a remarkable structure. When fully expanded it reaches a great 

 length, far exceeding that of the cell, and is supported by two hoop-like 

 thickenings or rings one more proximal, which is very distinct, and one 

 more distal, usually less distinct (Fig. 52, A and B, h}. At the base, up 

 to the first hoop, the collar is thickened and appears finely granular in 

 optical section ; beyond the first hoop it becomes much thinner, and its 



