Chap. IV.] CCELENTERATA. 1 09 



axial gastric cavity, with which there communicate 

 passages or canals. The stomach may be enlarged in 

 some, and diminished in other directions, and the 

 canals may be greatly developed in number, and pro- 

 vided with outgrowths or pouches ; but the essence of 

 the arrangement is still apparent. 



When, as so frequently happens, a number of 

 hydroid polyps become connected with one another by 

 a common trunk and form a colony, the gastric cavity 

 of each polyp is brought more or less into relation 

 with those of the rest ; for each cavity is continuous 

 with the canal which runs in the centre of the stem 

 or trunk of the colony and the cells which line this 

 passage are provided with cilia. The facts that some 

 polyps occupy positions moie easily accessible' to food 

 currents than others, and that the less fortunately 

 situated can draw on theii fellows, lead, in a number of 

 cases, to a division of labour , those best adapted for the 

 business of nutrition come to limit their activities to 

 this important duty (trophosomes), while others, 

 fed at their expense, devote themselves to the equally 

 important duty of developing the generative products, 

 and so take on the especial function of reproducing the 

 species (gonosomes). The Stylasteridae, on the other 

 hand, afford us examples of zooids which, having ceased 

 to be nutrient, have become reduced to mere tentacles, 

 the duties of which they alone perform (dactylo- 

 zooids). 



It is now necessary to direct attention to a portion 

 of the gastric apparatus of Hydra, which was, for the 

 moment, neglected ; the mouth of Hydra, or indeed of 

 any hydroid, is not a mere space in the wall of the 

 body, but forms a conical process, at the tip of which is 

 set the orifice, so placed that when a hydra is looked 

 at from the side, the mouth cone only can be seen, and 

 the wide mouth itself is hidden. 



If we pass now to the other extreme of the series 



