PORIFERA 



167 



2. Parts are arranged about ithe central cavity but usually 

 not in a symmetrical fashion. 



3. There are two distinct layers, dermal and gastral. These 

 are separated by a gelatinous middle region in which are included 



Fig. 76. 



Fig. 77. 



Fig. 76. Ltfucandra, a simple type of sponge. (From Delage and Herouard; "Traits de Zoologie 



Concrete.") 



Questions on the figure. — What is the position of the osculum? Which is the 

 attached end? How many individuals are represented in the cut? 



Fig. 77. Diagrams to illustrate the development of one of the simpler types of sponge: 1, the 

 egg; 2, section of 16- to 32-celled stage; 3, section of later stage, a ciliated larva (blastula) ; 4, gastriila; 

 5, section through older larva which has become attached by the end containing the blastopore. 



New openings break through by the coalescence and perforation of the two layers and a form 

 results such as is figured in Fig. 78. c, archenteron; bL, blastopore; ec, dermal layer; en., gastral 

 layer; mes., mesenchyma; s, segmentation cavity. 



Questions on the figures. — What terms would be applied to the cleavage and 

 gastrulation in this sponge? What is suggested as to the mode of forming meso- 

 derm? The attachment of the sponges by the blastopore end of the larva necessi- 

 tates what later development? See Fig. 78. Examine figures in other texts of the 

 development in other species. 



cells of different kinds {mesenchyma) not in a true layer. In the 

 cells of the dermal layer spicules are produced, forming the sup- 

 porting skeleton (Fig. 79, C). 



4. Non-sexual reproduction is prevalent, but dimorphic 

 sexual cells are also formed in the mesenchyma. The sexually 

 produced larva is free-swimming; the adult is attached. 



