METAZOA 



37 



PORIFERA. (the Sponges). 



Chaeactbeistics. — Animals of very varied size and sha^pe. 

 Nurnerous minute pores allow the passage of water into the 

 interior of the sponge, and the water is discharged through 

 larger openings known as Oscula. The current of water is 

 maintained hj certain flagellate cells, which are usually 

 aggregated in what are known as flagellate charnbers. The 

 mesoderm is ivell developed, and usually gives rise to a skeleton 

 of calcareous, siliceous, or horny material ; it also gives origin 

 to the reproductive cells. Sponges may be unisexual or 

 hermaphrodite. They are aquatic, and, ivith the deception of 

 the Spongillidae, they are marine. They are devoid of 

 tentacles and of nematocysts. 



The simplest type of Sponge is that of Ascetta primordialis, 

 described by Haeckel. It is a hollow vase-hke structure borne 

 on a stalk with its free end open. This 

 opening is the osculum. The walls are per- 

 forated by a series of small circular apertures, 

 the " pores," and its cavity is luied by a layer 

 of flagellate collared cells, whose activity 

 keeps a current of water entering the pores 

 and finding an exit through the osculum. 

 The flagellate cells are endodermal. The 

 outside of the vase-hke body is covered with 

 ectoderm, and between these two layers is a 

 mesodermic tissue which produces triradiate 

 calcareous spicules. The flageUate endoderm 

 cells are said to possess contractile vacuoles. 

 In the more highly organised Sponges the 

 endodermic Hning of the central cavity has 

 lost its flageUate character and become a 

 flat epithelium. 



Grantia compressa is a sponge with a 

 calcareous skeleton, which is frequ.ently met 

 with attached to rocks and stones round 

 our coast. It is of a whitish colour, seldom 

 more than an inch long, and rather variable in outline : 



Fig. 28. — Ascetta pri- 

 mordialis, Haeckel. 

 After Haeckel. 



