94 MANUAL OF ZOOLOGY SECT. 



is known that both blastostyles and medusa-buds are zooids, 

 so that the colony is trimorphic, having zooids of three 

 kinds. 



To make out the structure in greater detail, living speci- 

 mens should be observed under a high power. A polype 

 is then seen to consist of a somewhat cylindrical, hollow 

 body, of a yellowish colour, joined to the common stem by 

 its proximal end, and produced at its distal end into a 

 conical elevation, the manubrium or hypostome (mnb.\ 

 around the base of which are arranged the twenty-four 

 tentacles in a circle. Both body and manubrium are 

 hollow, containing a spacious cavity, the enteron (ent.\ which 

 communicates with the outer world by a mouth (mth.\ an 

 aperture placed at the summit of the manubrium. The 

 mouth is capable of great dilatation and contraction, and 

 accordingly the manubrium appears now conical, now 

 trumpet-shaped. Under favourable circumstances small 

 organisms may be seen to be caught by the tentacles and 

 carried towards the mouth to be swallowed. The hydro- 

 theca (h.th.) has the form of a vase or wine-glass, and is 

 perfectly transparent and colourless. When irritated by 

 a touch, or by the addition of alcohol or other poison the 

 polype undergoes a very marked contraction : it suddenly 

 withdraws itself more or less completely into the theca, and 

 the tentacles become greatly shortened and curved over the 

 manubrium (P. 2). 



The various branches of the common stem show a very 

 obvious distinction into two layers : a transparent, tough, 

 outer membrane, of a yellowish colour and horny consist- 

 ency, the perisarc (/.), and an inner, delicate, granular 

 layer, the coenosarc (cot.), continuous by a sort of neck or 

 constriction with the body of each hydranth. The coenosarc 

 is hollow, its tubular cavity being continuous with the 



