METAZOA 



41 



diaphragms in the exhalent canals of the flagel- 

 late chambers are believed to be muscular, and 

 a nervous nature has been attributed to others 

 situated near the inhalent pores. 



The skeleton of Grantia consists of cal- 

 careous spicules, most of which are of a 

 triradiate shape (6, 7, and 8, Kg. 29). Each 

 spicule commences to appear in one of the 

 mesodermal cells, but as it increases in size it 

 may protrude from this. The spicules are said 

 to gradually work towards the exterior of the 

 sponge, and to be discharged as waste matter. 



The character of the skeleton is made use 

 of in classification. Only a very few sponges — 

 Halisarca, Oscar ella, and Chondrosia — are devoid 

 of any kind of supporting structure. Those 

 which possess calcareous spicules have been 

 grouped together as the Calcarea, and opposed 

 to all the other sponges, or Non-calcarea. The 

 skeletons of this latter group may consist of 

 siliceous spicules, or of a fibrous substance — 

 spongin. The siliceous, like the calcareous 

 spicules, originate in a single mesodermic cell 

 (Fig. 32) ; both assume a great variety of size and 

 shape, and the former may be articulated or 

 fused to one another. The organic skeleton, 

 found in the Order Cornacuspongiae, consists of 

 spongin, a substance chemically allied to silk. 

 It is secreted by a number of mesodermic cells 

 termed spongoblasts, which form a layer all 

 round the fibre, and a multicellular cap covering 

 the ends (Fig. 34). The fibrous skeleton of Bu- 

 spongia, devoid of spicules, and characterised 

 by the regular arrangement of the network and 



the smallness of the meshes, forms the bath 

 Fig. 32. — Spicules « 



originating in sponge of Commerce. 



single cells. Jq Qrantia some of the amoeboid mesodermic 



After SoUas. 

 I. From Stelletta. II. From embryo of Craniella cranium. III. The same from 

 adult Craniella cranium. IV. A four-rayed spicule from Theonella Simnhoei. 



II 



III 



