54 



SYSTEMATIC ZOOLOGY 



into germ-cells, while in the more highly organized members of 

 the type there are organs of sight and hearing. Very many of 

 the Coslenterata are attached to foreign objects, and often form 

 large colonies by budding. 



While the animals belonging to this type present the charac- 

 teristics mentioned above, they form, nevertheless, two groups 

 which are otherwise pretty distinct from one another; these 

 groups constitute the subtypes. 



SUBTYPE I. PORIFERA 



The Porifera ( Lat. poms, a pore, and ferre, to bear), or 

 sponges, are, in their simplest condition, hollow cylinders 



attached 

 open at 

 opening 

 osculum 



at one end and 

 the other ; the 

 is known as the 

 (Fig. 37). The 

 walls of the cylinder are 

 pierced by a large number 

 of canals, which put the 

 central cavity in communi- 

 cation with the outside ; 

 the openings of these 

 canals on the surface are 

 called the lateral pores. 

 The exterior of the sponge 

 is covered with ectoderm, 

 the interior and the lateral 

 canals in part are lined 

 with entoderm. In some 

 species the whole entoderm 

 consists of collared, flagel- 

 late cells, preciselv like the 

 individuals of the Choano- 

 flagellata ; and in other 

 species these collared cells 

 are confined to the canals 

 or parts of the canals (Fig. 38). The water, bearing particles of 

 food, passes through the pores and canals, and the flagellate cells 



FlG. 37. Sycon gelatinosum. Portion of a colony, 

 enlarged; one individual bisected longitudil 

 showing coelenteion within, and lateral i 

 through the w osi linn; //. three of the 



lateral pores. (After Parker and Haswe 



