100 



COLLEGE ZOOLOGY 



axon (f). Spicules with three rays like most of those in Leuco- 

 solenia and Grantia are called triradiate. The skeletons of the 



I I 



41 T V 



FIG. 59. Types of canal systems of sponges. A, Ascon type. B, Sycon 

 type. C, Rhagon type (Spongilla). The arrows indicate the direction of the 

 current of water. The thick black line in A and B represents the gastral layer ; 

 the dotted portion, the dermal layer, ap.p, apopyle; fl.c, flagellated chamber; 

 GC, gastral cavity (cloaca); in.c, incurrent canal; osc, osculum; pr.p, pro- 

 sopyle. C, flagellated chambers ; DP, dermal pores ; Ex, excurrent canals ; 

 GO, openings of excurrent canals; In, incurrent canals; O, osculum; PG, gastral 

 cavity; SD, subdermal cavity. (A and B, from Minchin in Lankester's 

 Treatise; C, from Parker and Haswell, after a Leuckart-Nitsche wall-chart.) 



horny sponges, of which the common bath sponge is an example, 

 are made up largely of fibers of spongin (Fig. 61). This sub- 

 stance, which is chemically allied to silk, is secreted by cells of 

 the dermaJ'l'ayer 



