CAMPANULARIAN HYDROID. 53 



distinct in a young specimen, although in an old specimen 

 it is difficult to make out any thing more than their 

 nuclei. 



- 2. Around the circumference of the umbrella there is 

 usually a prominent ridge (Fig. 26, 6) produced by the 

 folding back of the tentacles. 



3. The marginal tentacles (Fig. 26, c) are rather sharply 

 divided into an enlarged broad bulb (Fig. 26, d) and a 

 more slender cylindrical, slightly tapering shaft, e. In 

 the shaft notice : — 



(i.) The ver^' thin layer of ectoderm, which is thick- 

 ened at intervals to foiTu annulations which are tilled with 

 larofe nematocvsts. 



(ii.) The longitudinal muscular fibres which lie under- 

 neath the ectoderm. 



(iii.) The ti'ansparent supporting layer. 



(iv.) The solid axis of large endoderm c«Ils. 



4. In the bulb at the base of the tentacle, notice : — 



1. The thickened layer of large prominent rounded 

 ectoderm cells. 



2. A large central endoderm cell. 



b. The sub-umbrellar surface (Fig. 27). 



1. The vase-shaped manubrium (Fig. 27, a) with a 

 w ide opening, the margins of which are divided into four 

 lobes. 



(i.) The line of nematocysts which fringes the mouth. 

 (ii.) The polygonal ectoderm cells which cover the 

 manubrium. 



2. A nearly square stomach-chamber (Fig. 27, 5) which 

 lies in the centre of the sub-umbrella, and is separated by 

 a somewhat contracted neck from the cavity of the manu- 

 brium. 



3. The four radiating chymiferous tubes (Fig. 27, c) 



