54 



SPONGES 



amongst which the species Ascandra 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 again 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, B, c). The 

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

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

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

 length, and the distal third forms a freely projecting " neck :} (collum\ 



Flo. 52. 



Collar cells of various sponges. .-1, of Ascandra falcata, H. J5, of Clathrinn coriacea, Mont. ; 

 a, fully expanded ; />, less expanded ; c, retracted down to hoop ; d, condition of complete 

 contraction. C, a, collar cell of Sycon ciliatum, Fabr. ; h, of Leticosolenia complicata, Mont. 

 D, a, collar of HalU'homlrin }xtnic'ea .- b, of Spongilla ; be, base of collar ; Col, collar ; Jl, 

 flagellum ; h, hoops rapporting collars ; n, nuclei. C, a, after Bidder ; D, a and b, after Vosmaer 

 and Pekelhariny. D, x 1000 ; A-C, x about 850 or 900. 



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

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

 of protoplasm continuing the neck, from the centre of which arises the 

 flagellum. 



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 flagellum, representing very probably 

 food vacuoles, by means of which the cell ingests food particles captured 

 by the flagellum. Contractile vacuoles have been frequently described 

 by older authors (e.g. James-Clark, the discoverer of the true nature of 



