92 MICROSCOPICAL STUDIES. 



of each hydrotheca is somewhat narrowed and sculptured into points 

 mucronate. The body of each poly pi te is nearly cylindrical 

 (compare with the cup-shaped body in Obelia) and has a special 

 bundle of retractile fibres inserted on the outer surface of the body, 

 some little way beneath the tentacles on the side turned towards 

 the axis of the stem. Thence they pass inwards and slightly down- 

 wards to become attached to the wall. of the hydrotheca where fused 

 with the stem perisarc. By contraction these fibres energetically 

 assist in the retraction of the polypite. 



The body wall of each polypite, as also the coenosarcal tube, 

 consists essentially of a simple ectoderm layer separated from a 

 ciliated endoderm by a delicate supporting lamina. 



The endoderm is usually separated by a space from the investing 

 perisarc, except at the base of each hydranth and at the points where 

 growth is taking place. 



The perisarc is a secretion of the ectoderm and thus, where a 

 polyp bud or a new branch is originating, the ectodermal cells are 

 of enormous size. As the perisarc is completed the ectodermal cells 

 dwindle and shrink away from it. 



The special growth of the branch is as follows : the perisarc 

 between the two terminal hydrothecse becomes absorbed and the 

 blind termination of the ccenosarc pushes through ; next, this throws 

 out a hollow bud on either side. In these three buds, the ectoderm 

 is very thick and active and rapidly forms a layer of perisarc, the 

 lateral buds becoming hydranths, the median, an internode of the 

 hydrocaulus or stem. 



The branches bear in their number a direct ratio to the 

 abundance of nutriment and other favourable life conditions ; when 

 present, they have always one definite point of origin, and that is, 

 from the hydrocaulus just beneath the base of a hydrotheca. There 

 the perisarc is absorbed and the ccenosarc pushes out a tiny bud 

 which in further development repeats the process of apical growth 

 already described. 



The tentacles have the same structure as in Obelia. 



Eepr eduction. In the breeding season numerous large ovate 

 sacs the gonothecaB or gonangia appear here and there on the 

 main stems and branches. Their origin is similar to that of the 

 branches, by absorption of the perisarc and thrusting out of a hollow 

 bud of coenosarc. These sacs produce in due course the reproductive 

 elements. 



The sexes are separate and in separate colonies. Male gonangia 

 can usually be distinguished from female by their shape the 

 former being regularly oval, the latter irregularly ovate. 



