54 



SPONGES 



amongst which the species Ascundra falcata, H., is pre-eminent in this 

 respect, and may be taken as a type (Fig. 52, A\ The cells in (question 

 are columnar, and about half as long apain as they are broad in the 

 fully expanded state. When contracted they become narrower and more 

 elongated, a change due to pressure of the surrounding tissues, and not 

 probably to the activity of the collar cells themselves (Fig. 52, i>, c). The 

 large nucleus is lodged at the base of the cell, as is always the case 

 in Clathrinuiae, at least during the resting state of the cell. Each 

 choanocyte is in contact with its neighbours fur about two-thirds of its 

 length, and the distal third forms a freely projecting " neck " {cAlum), 



Collar cells of \-arious sponites. 

 n, fully expanded ; '■. !►•>■; -xj^n." 

 contraction. C. a. ■ 

 D, a, collar of Hn' 

 lUgellum ; A, hoops s _ 

 and Pekelhariug. D, xluoO; a-c, x about &50 or 900. 



Fi.i. 52. 



A, oX Ascandrn falcata, H. /?, of Clathrina. ayriaetf, Mont. ; 



■■i ". c. retracteil down to hoop: 'I. o.ni.litiin of complete 



•'iliatum, Fabr. ; h, of / ' i, Mont. 



'i, of :>pongilla ; he. hi- 'll.ir ; _/f, 



•I, nuclei. C, a, after Bi'ii ■ Vosma'er 



bearing the collar (coUatt). The junction of body and neck is marked by 

 a distinct flange or "shoulder." The base of the collar encloses a mound 

 of protoj)lasm continuing the neck, from the centre of which arises the 

 tlagellum. 



The cytoplasm has a very distinct alveolar or vacuolar structure, and 

 larger vacuoles — or it may be, one large vacuole — are commonly found at 

 the upper extremity, just below the tlagellum, representing very probably 

 foixl vacuoles, by means of which the cell ingests food particles captureii 

 by the flagellum. Contractile vacuoles have been fretiuently described 

 by older authors («v/. .Tanu->-r'I.irk. tlio discoverer of the true rature of 



