92 MANUAL OF ZOOLOGY sect, iv 



found chiefly towards the proximal region of the colony, are 

 long cylindrical bodies or blastostyks (bis), each enclosed 

 in a transparent case, the gonangium or gonotheca (gih), 

 and bearing numerous small lateral offshoots, varying greatly 

 in form according to their stage of development, and known 

 as medusa-buds (m.bd). By studying the development of 

 these structures, and by a comparison with other forms, it 

 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 hol- 

 low, containing a spacious cavity, the enteron (ent), which 

 communicates with the outer world by a mouth (frith), 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 (hth) 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). 



