312 



ORGANS OF DIGESTION. 



FIG. 108. 



whicn lead obliquely upwards through the parietes to the 

 general internal cavity, and from this cavity the currents 

 pass out by an inferior orifice (Fig. 3 d.), the upper part 

 (Fig. 3. c.) in such forms being the closed part of attach- 

 ment. The proper vents in such tubular species are there- 

 fore only seen upon opening the parietes and observing the 

 inner surface (Fig. 3 b.} which is entirely covered; with 

 orifices of a larger size than the exterior pores. In the 

 branched forms of poriphera, which, from the softness of : 

 their texture, appear always to hang downwards from their 

 point of attachment, the whole outer surface is closely 

 studded with minute pores, as in the haliclona oculata (Fig. 

 2 A. .), and from these pores the anastomozing canals 

 wind through the interior to open on the margins of the 

 branches by wide prominent vents (Fig. 2 b. b.). The 

 vents are disposed in all the different forms of porifera so as 

 least to incommode the absorbent pores by the flocculi of 

 matter constantly discharged from them with the currents. 

 The vents are raised from the general absorbent surface to 

 the ends of projecting papilke (Fig. 108. b. b. c. c.) in those 

 species which are attached to the acclivities of rocks ; there 



