ECHINODEEMS ASD BEYOZOOA. 165 



o varies (t), which open by canals passing through each of the 

 five ovarial plates. The auricles surrounding the mouth (i) 

 give attachment to the lantern ; the ambulacral avenues (e) 

 give passage to tubular feet ; the simple spines () arming 

 shell are moved by muscles ; the small trident spines, or 

 pedicellariae (), move like forceps, and the long tubular feet 

 (e) are protruded by the injection of a fluid ; an oblong vesi- 

 cle (/) opens near the mouth ; the intestine is retained in situ 

 by a delicate mesentery (p), on which blood-vessels ramify ; 

 currents of water flow constantly through the shell, their course 

 being directed by the vibratile cilia covering the lining mem- 

 brane of the test ; the net-work of blood-vessels ramifying 

 upon these membranes is therefore bathed by the sea-water, 

 and maintained in a state of oxygenation, so that the whole in- 

 terior of the shell of urchins is a great respiratory chamber. 



In the Holothuria (fig. 232) the long and uniform intestinal 

 canal makes several convolutions before terminating in the 

 cloaca; around the mouth are numerous caecal salivary ves- 

 sels ; a mesentery retains the intestine, and affords an ex- 

 tensive surface for the ramification of blood-vessels ; the re- 

 spiratory tubes are distinct from the general cavity of the 

 body, and form an arborescent organ like a rudimentary 

 lung. 



[ 317. In the BRYOZOOAX POLYPIFERA, as the Pluma- 

 tella (fig. 175), the digestive organs present a much higher 

 phase of development than in the hydraform group, and mani- 

 fest an approach to the type of the tunicated mollusca. The 

 mouth is surrounded by a circle of ciliated tentacula, the vibra- 

 tions of which cause currents of water to flow towards the 

 oral aperture ; the possession of ciliated tentacula forming one 

 of the distinctive features of this group. The mouth, situated 

 in the centre of the tentacular circle, leads into a long saccu- 

 lated stomach, the walls of which are studded with glandular 

 specks, or biliary follicles. From about the middle of the 

 stomach the intestine proceeds, and ascending close to its walls, 

 opens by a rectum near the mouth (c), in such a position that 

 the excrementitious matter ejected therefrom is at once carried 

 away by the currents sweeping round this region ; the in- 

 testinal canal is attached to the sac by muscular bands, and 

 floats freely in the visceral cavity. The tegumentary sheatn. 

 is an organic portion of the polype, and, after enclosing 



