Ill PORIFERA 121 



more or less distinct though closely apposed cells^ formed by the cytoplasm 

 of the original scleroblast clumping together round the two nuclei. As 

 the two spicule-forming cells keep on depositing more and more calcium 

 carbonate on the spicule they wander apart^ one passing towards the tip 

 of the ray, the other remaining at its base. Eventually the apical cell 

 disappears and after a time the basal cell wanders out and takes its place 

 at the tip of the ray (Fig. 55, sc). 



One of the rays of the triradiate spicule commonly differs in length 

 from the other two, the whole spicule having a Y-shape. The spicule 

 has a definite orientation, the unpaired ray, forming the stem of the Y, 

 being arranged longitudinally and pointing away from the osculum 

 (Fig. 54). In accordance with the cylindrical form of the wall of the 

 sponge in which they are embedded the two equal rays are not exactly 

 in the same plane and they may be slightly curved. 



Of the cells which leave the dermal epithelium and wander into the 

 jelly a third set are characterized by their retaining a tb-all-appearance 

 undifferentiated amoeboid character. These amoebocytes (Fig. 55, a), 

 which are found scattered irregularly through the jelly, play an important 

 part in the life of the sponge. They serve for the transport of food and 

 excretory material, and it is some of them that function as the gonad. 

 In the latter case the cell rounds itself off and either divides over and 

 over again to form a mass of microgametes, or simply increases in 

 size and stores up reserve food material or yolk, becoming a single egg or 

 macrogamete. 



The above description has dealt with the genus Leucosolenia but young 

 Ascons collected for practical work will often be found to belong to another 

 genus — Clathrina. While agreeing in its main features with Leucosolenia, 

 Clathrina presents certain differences in detail which serve for its identifi- 

 cation, (i) The individual tubes and their branches tend to undergo 

 fusion together so as to form a kind of network ; (2) the nuclei of the 

 choanocytes are close to their basal or attached end ; and (3) all three 

 angles between the rays of the three-rayed spicules are equal. 



Grantia 



The Ascon is one of the simplest types of Sponge. A good- idea^of 

 the way in which sponge structure becomes more complicated is obtaiiied 

 by the study of another very common type of sponge — ^the Syeon type — 

 exemplified by the genus Grantia (Fig. 56). While built up of jDrecisely 

 the same elements as the Ascon, the Sycon differs in its usually larger 

 size and more complicated arrangement of these elements. Simple 



