18 SPONGES 



(ii) Anchor spicules, having the form of an anchor, 

 (iii) Amphidisc spicules, consisting of a rod bearing 



at the distal 



Fig. 5.— Types of Sponge Spicules. After J. Arthur Thomson. 

 1, Monaxon ; 2, triradiate ; 3, triaxon ; 4, tetraxon ; 6, anchor ; 6, polyaxon ; 7, an amphidisc. 



extremities disc-like expansions curved towards the centre and 

 prolonged into several tooth-like protuberances. 



The Main Types of Canal-system, 



All the cavities of the body traversed by the currents of water which 

 nourish the sponge are termed collectively the canal-system. This system 

 attains a high degree of complexity, and four different types may be distin- 

 guished according to the situation of the choanocytes : 



1. The Ascon Type. 



The canal-system is simple. The water enters through the numerous 

 temporary inhalent pores in the body-wall into the central gastral cavity of 

 the tube, at the top of which is the exhalent osculum, through which the 

 water passes out. The choanocytes line the gastral cavity. Examples : 

 Leucosolenia, Clathrina (Class, Calcarea ; Order, Homocoela). {See Fig. 6.) 



2. The Sycon Type. 



This type arises from the Ascon type by a process of outgrowth and fold- 

 ing of the body- wall, resulting in the formation of a number of blind diverti- 

 cula of the gastral cavity and the restriction of the choanocytes to the interior 

 of these diverticula, which are therefore called the flagellated or radiating 

 chambers (radial canals). The gastral cavity is lined by flattened epithelium 

 derived from the dermal layer. Between the radiating chambers canals are 

 enclosed, from which the water flows into the chambers. These are called 

 the incurrent or inhalent canals, and are lined by the flat epithelium of the 



