TYPE CaSLENTERA. 



79 



SO that the free end has a somewhat conical shape, the month 

 being situated at the end of the cone. At the base of the 

 cone, which is termed the hypo- 

 stome, there is usually a circle of 

 tentacles, though occasionally 

 they are scattered irregularly over 

 the surface of the hypostome, and 

 a second circle of tentacles may 

 be present at the base of the 

 polyp (Pennaria). In colonial 

 forms each individual (hydranth) 

 (Fig. 39, hy) is situated at the ex- 

 tremity of a stalk or hydrocauLus 

 (he), the various stalks either 

 uniting together to form a branch- 

 ing colony or else arising from a 

 network, the hydrorhiza, which 

 covers the surface upon which 

 the colony grows ; or occasion- 

 ally each hydranth arises, with- 

 out the intervention of a hydro- 

 caul us, from a flat plate-like 

 expansion common to all. In 

 either case the coelenteron is 

 continuous throughout the entire 

 colony, the fleshy substance of 

 the hydrocaulus and hydrorhiza, 

 coenosarc (co), being tubular and in 

 direct continuity with the body- 

 walls of the hydranths. The 

 coenosarc is enclosed within a chitinous substance termed the 

 perisarc (jp) secreted by the ectodermal cells and sometimes 

 prolonged at the end of each hydrocaulus into a cup-like 

 structure, the hydrotheca (ht), into which the hydranth may 

 be retracted ; occasionally this ectodermal secretion takes 

 the form of carbonate of lime. 



The ectoderm generally shows a considerable amount of 

 differentiation of its constituent cells. In addition to the 

 cnidoblasts, which have been already described (p. 77), epithe- 



FiG. 39.— Portion op a Colony 



OF THE CAMPANTXLAEIAN HT- 



DROiD Olyiia. 



CO = coeuosarc. 

 he = hydrocaulus. 

 ht = hydrotheca. 

 hy = hydranth. 

 go = gonotheca. 

 m = mouth. 

 p = perisarc. 

 t = tentacle. 



