i66 



ZOOLOGY 



2. Parts are arranged about the central cavity but not usu- 

 ally in a symmetrical fashion. 



3. There are two distinct layers, ectoderm and entoderm. 

 These are separated by a gelatinous middle region in which are 

 included cells of different kinds (mesenchyma or mesoderm) 



FIG. 76. 



FIG. 77- 



FIG. 76. Leucandra, a simple type of sponge. (From Delage and Herouard; "Traitede 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: i, the 

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

 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 ectoderm and entoderm, 

 and a form results such as is figured in Fig. 78. a, archenteron; &/., blastopore; ec., ectoderm; ., 

 entoderm; 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. 



not in a true layer. In the cells of the mesenchyma spicules 

 are produced, forming the supporting 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. 



