SIMPLE SPONGES 83 



animals, using the principal opening not for intaking — as a mouth 

 — but for casting out. The wall of the vase consists of two layers, 

 a gastral layer, composed of collared 

 flagellate cells, or choanocytes, standing 

 side by side but not touching, which 

 lines the internal cavity or paragaster 

 except for a short distance within the 

 rim ; and a dermal layer, which makes 

 up the greater part of the thickness of 

 the wall and is turned in a little way at 

 the rim. This layer again consists of two 

 parts, a covering layer of flattened cells, 

 known as pinacocytes, rather like those 

 of a pavement epithelium, but with the 

 power of changing their shape, and the 

 skeletogenous layer, between the cover- 

 ing layer and the gastral layer. The 

 skeletogenous layer consists of scattered 

 cells, embedded in a jelly of collagen. ^^ ^ ., . 



^, , -V A, n Fig. 49.— Choanoflagellata. 



The most numerous of these cells are ^^ ^^^^^,^^^ t, saipin,.ca; 



engaged in secreting spicules of calcium cPoiyaca. 



carbonate by which the wall is supported. 



They wander from the covering layer into the jelly, and then 



each divides into two, and the resulting pair secrete in their 



protoplasm, which is continuous, 

 a needle-like spicule which pre- 

 sently outgrows them. Most often 

 the original spicule cells come 

 together in threes before this 

 process, so that the three spicules 

 which they secrete become the 

 rays of a three-rayed compound 

 spicule. This lies in the wall with 

 two rays towards the osculum 

 and one away from it. Some- 

 times a fourth cell joins the 

 others later, and forms a fourth 

 ray which projects inwards to- 

 wards the paragaster. Often there 



are simple spicules which project from the surface of the sponge. 



Other cells, known as porocytes, of a conical shape, extend 



Fig. 50. — Proterospongia. 



a, Nucleus of a collared member of the colony ; 

 b, contractile vacuole ; c, amoeboid member ; 

 d, division of amoeboid member ; e, flagellate 

 member with collar contracted ; /, jelly ; 

 g, member forming spores. 



