386 CORYMOEPHIDiE. 



One of the most striking features in the trophosouie of ClmatcUa prolifera is the compara- 

 tively great development and the elongated form of the hydranth, which, just as in Clava, 

 contrasts strongly with the rudimental and inconspicuous hydrocaulus. When extended its 

 form is that of a cylinder, slightly dilated at its proximal end, where it springs from the summit 

 of its supporting hydrocaulus, and at its distal end, where it carries its verticil of tentacles. 

 When contracted its form varies, according to the degree of contraction, from that of an elongated 

 cone to that of a flask. 



The tentacles are usually seven or eight in number, springing in a single verticil from the 

 dilated summit of the hydranth, and surrounding a short bluntly conical hypostome. When 

 extended they slightly taper from their base towards their summit, where they terminate in a 

 well-defined spherical capitulum, formed by an accumulation of large thread-cells in the thickened 

 ectoderm. 



The fully formed planoblast is very remarkable in the non-development of a free umbrella. 

 It is thus destitute of an organ of natation ; but, to compensate for this deficiency, the marginal 

 tentacles are peculiarly developed and adapted to the function of creeping, so that the free 

 gonophore of Clavatella, instead of being natatory, like other hydroid medusae, is truly ambulatory. 

 The generative elements are produced between the endoderm and ectoderm on the dorsal 

 or proximal side of the medusa. In all the specimens I have examined the medusa had six 

 marginal tentacles. The variation from this number noted by Filippi, who occasionally found 

 seven, is probably abnormal. The planoblast may frecpiently be seen repeating itself by buds, 

 which are developed in the interradial spaces of the margin. 



Clavatella prolifera is a singularly beautiful little hydroid. It grows in small scattered 

 groups, which the eye will easily detect in the shallow reservoirs of clear water left behind by the 

 retiring tide, and whose gracefully bending hydranths, with their coronals of globe-tipped 

 tentacles, and budding clusters of medusae, render it one of the most attractive and interesting of 

 all the smaller tenants of the rock-pool. 



It appears to be confined to a zone just below the upper limit of the range of neap tides, 

 and seems to prefer the smaller and shallower pools, more especially such as have their sides 

 overgrown with an incrustation of nullipores. 



CORYMORPIIIDjE. 



TROPHOSOME. — Hydrocaulus solitary, destitute of perisarc. Hydranths with 

 a proximal and a distal set of filiform tentacles. 



GONOSOME. — GoNOPnoKES in the form of medusiform planoblasts, with four 

 radiating canals, and one or more simple marginal tentacles. 



