ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



tcosolenia frequently undergo an imperfect kind of budding, 



ring out into tubular pockets which radiate out at 



axis of the sponge. Now in a typical Sycon the 



U J 1(] is u. S et lv siu-h outgrowths in close contiguity with one 



57, B). At their tree ends the outgrowths are fused more 

 ] ir ,, as to present a continuous surface broken by small 



openings whirl, lead into the spaces between the original outgrowths, 

 eing m>\\ known us the inhalent canals (Fig. 57, C, i.c). 

 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- 

 in- 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. 



The PORIFERA are, with the exception of a 

 few genera, marine in habit. They vary greatly in 

 si/.r and form the differences in general appearance being due 

 1 1 part to imperfect processes of budding and fission: e.g. 

 may reach ;l relatively large bulk and numerous oscula 

 us surface betoken so many incompletely separated 

 imlividiiaU. Internally the differences have to do mainly with differences 

 . which are referred to collectively as the canal-system. In 

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

 the central cavity and the radiating chambers to which 

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



'itaining the choanocytes become small and rounded 

 i while their communications with inhalent canals and with central 

 me drawn out into more or less complicated tubular channels. 



i. X6. 



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