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 tlagellum is long, slender, and of even thickness throughout 

 its length. It arises in Clathrinidae always from 

 a distinct granule of peculiar staining pro]>erties, 

 situated at the summit of the cell. When the cell 

 is fully expanded, the tlagellum in preparation 

 ajjpears 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 



Pi 



,-r^ 



.fA 



Fig. 53. 



Choanoflagellata, after France. On the left, Codonosiga hotrytis, J. CI., x3o0, showing the 

 comuiPncinK transverse fission. In the middle, Salpingoeca fuHformis, S. K., x500. On the 

 right, Dii'hisi'id EnUii, France, x 4C0. col, collar ; i.col, e.col, in Diplosi'ja, internal and external 

 collar ; .//, tlagellum ; st, stalk ; th, tlieca. 



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

 with a longer pause on one side than on tlie 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 luovement, each one 

 moving (piite independently of its neighbours (Vosmaer and Pekelharing 

 [30]). 



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

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

 length, far exceeding that of the cell, and is supported by two hooji-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 hoo[i, the collar is thickened and appears finely granul.ar in 

 optical section ; beyond tlie first hoop it Ijecomes much thinner, and its 



