ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Ascons like Leucosolenia frequently undergo an imperfect kind of buddings 

 the body-wall growing out into tubular pockets which radiate out at 

 right angles to the long axis of the sponge. Now in a typical Sycon the 

 whole surface is beset by such outgrowths in close contiguity with one 

 another (Fig. 57, B). At their free ends the outgrowths are fused more 

 or less together so as to present a continuous surface broken by small 

 openings which lead into the spaces between the original outgrowths, 

 these spaces being now known as the inhalimt canals (Fig. 57, C, i.c). 

 These canals, being as it were parts of the outer world enclosed between 

 the outgrowths, are lined with ordinary dermal epithelium similar to 

 that covering the rest of the outer surface. The pores 

 in the Sycon are restricted to the walls of the inhalent 

 canals and are consequently not visible when the 

 sponge is viewed from the outside (Fig. 57, C). The 

 interior of the Sycon is divided into a central cavity 

 {c.c) and radiating chambers {r.c), the latter represent- 

 ing the cavities of the outgrowths. Finally the 

 choanocytes are found only in the radiating chambers, 

 the central cavity being lined with a simple thin 

 epithelium like that covering the external surface. 



Apart from the differences that have been men- 

 tioned the Sycon type of sponge agrees with the 

 Ascon : here again the body-wall is composed of the 

 same elements — gastral layer with its choanocytes, 

 and dermal layer with its jelly, its dermal epithelium, 

 its porocytes, amoebocytes and scleroblasts. 



Fig. 56. 



Grantia. x 6. 

 OS, Osculujn. 



The PoRiFERA are, with the exception of a 

 few genera, marine in habit. They vary greatly in 

 size and form — the differences in general appearance being due 

 in great part to imperfect processes of budding and fission : e.g. 

 the sponge may reach a relatively large bulk and numerous oscula 

 scattered over its surface betoken so many incompletely separated 

 individuals. Internally the differences have to do mainly with differences 

 in the spaces, which are referred to collectively as the canal-s.vstem. In 

 the Aseons there is just the single gastral cavity (Fig. 57, A) ; in the 

 SyCons there is the central cavity and the radiating chambers to which 

 the choanocytes are restricted (Fig. 57, B and C) ; in still other sponges 

 the chambers containing the choanocytes become small and rounded 

 in form while their communications with inhalant canals and with central 

 cavity become drawn out into more or less complicated tubular channels. 



