CELLS AND SPICULES 



171 



rent and incurrent canals. The flagellated chambers are lined 

 by " choanocytes " (cf. Fig, 70, p. 170); these are cells provided 

 at their inner end with a tlagelluni and a collar surrounding 

 it. They resemble individuals of the Protozoan sub -class 

 Choanoflagellata, and the likeness is the more remarkable because 

 no other organisms are known to possess such cells. Taken 

 together the choanocytes constitute the " gastral layer," and they 

 are the active elements in producing the current. The tissue 

 surrounding the chambers thus lying between the excurrent 

 and incurrent canals consists of 

 a gelatinous matrix colonised by 

 cells drawn from two distinct 

 sources. In tlie first place, it 

 contains cells which have a 

 common origin with the pina- 

 coeytes, and wdiich together with 

 them make up the " dermal 

 layer " ; these are the " collency tes " 

 and " scleroblasts " ; secondly, it 

 contains " archaeocytes," cells of 

 independent origin. 



Collencytes are cells witli 

 clear protoplasm and thread-like 

 pseudopodial processes ; they are 

 distinguished as stellate or bipolar, 

 according as these processes are 

 many or only two. Scleroblasts 

 or spicule cells are at first rounded, but become elongated with 

 the growth of the spicule they secrete, and when fully grown 

 are consequently fusiform. 



Each spicule consists of an organic filamentar axis or axial 

 fibre around which sheaths of silica hydrate are deposited succes- 

 sively by the scleroblast. Over the greater length of tlie spicule 

 the sheaths are cylindrical, but at each end they taper to a point. 

 The axial canal in which the axial fibre lies is open at both ends, 

 and the fibre is continuous at these two points with an organic 

 sheath, which invests the entire spicule. From this structure 

 we may conclude that the spicule grows at both ends — i.e. it grows 

 in two opposite directions along one line — it has two rays lying in 

 one axis, and is classed among uniaxial diactinal spicules. Being 



\i>. — Diagrammatic section of a 

 siliceous Sponge, a.p, Apopyle ; d.o, 

 dermal ostia ; ex.c, excurrent, or ex- 

 halant canal ; in.c, inenrrent canal ; 

 V, osculum. (Modified from Wilson.) 



