SPONGES, FLAT WORMS, AND ROUND WORMS 151 



by indefinite masses of spongy tissue. This makes it difficult to 

 decide what constitutes an individual sponge. Perhaps the best 

 way to separate one from another is to consider all of the tissue 

 surrounding one osculum as a single individual. 



c.c. 



B 



FIG. 75. Types of canal systems of 

 sponges. The arrows indicate the 

 direction of the current of water. 

 The thick black line represents the 

 gastral layer, the dotted portion, the 

 dermal layer, ap. p., apopyle ; 

 ex. c., excurrent canal ; fl. c., 

 flagellated chamber ; G C, gastral 

 cavity (cloaca) ; in. r., incurrent 

 canal ; osc., osculum ; ost., ostia ; 

 pr. p., prosopyle. (After Minchin 

 in Lankester's Treatise.) 



Canal System. The canal 

 systems of sponges may be 

 grouped under three types; the 

 first (Fig. 75, A) consists of in- 

 current pores (/>), a gastral cavity 

 (GC), and an osculum (osc.}. 



