THE HYDROMEDUSAE u 



sensory cells provided with long sensory hairs (Cunina), or becomes 

 surrounded by a closed vesicle, a stage in the formation of which 

 is shown in Fig. 14 j the club is supported in position by 

 long sense hairs, and the vesicle filled with fluid (Rhopalonema). 

 The endoderm, in secreting the otolith, has here a definite sen- 

 sory function, which is confined to the ectoderm in ocelli and 

 otocysts. 



Gland cells, pigment cells, cnidoblasts, and supporting or interstitial 

 cells are of constant occurrence ; the cnidoblasts are especially well 

 developed on the tentacles. 



The endoderm has much the same characters as in the hydroid. 

 The mesogloea is often extremely thick, especially on the exumbral 

 surface. Although it is essentially a non-cellular layer, but is 

 rather an inert secretion by ectoderm and endoderm, when well 

 developed, it often contains amoeboid wandering cells (Fig. 8 8 ), and 

 elongated muscle cells, both in all probability migrants from the 

 ectoderm. It is firm and jelly-like, and often shows a fibrillated 

 structure. 



The ova and spermatozoa are, with rare exceptions, of the 

 type usual in the Animal Kingdom. 



ORDER 1. Anthomedusae (Gymnooiastea). 



DEFINITION. Hydromedusae with a regular alternation (meta- 

 genesis) of a sterile hydroid generation with a sexual generation 

 of medusoids or other gonophores. The perisarc does not form 

 hydrothecae into which the hydroids are completely retractile, nor 

 rigid permanent gonothecae. The sense organs of the medusoids 

 are ocelli. The generative organs lie in the wall of the 

 manubrium. 



The HYDROID is colonial and fixed (Bougainvillea) ; or is non- 

 colonial, and then is either fixed (Myriothela) or free (Hydra). 

 The hypostome is conical or truncated, rarely trumpet-shaped 

 (Eudendrium). The tentacles are hollow (Hydra) or, more usually, 

 solid (Bougainvillea) ; they are rarely absent (Microhydra). They 

 are irregularly scattered (Hydra), or form a circlet (Bougainvillea), 

 or even two circlets (Tubularia) round the mouth. They never 

 have a pore at the tip, and are rarely branched (Cladocoryne) or 

 webbed. They are filiform (Hydra) or capitate (Coryne); in 

 the former case the nematocysts are chiefly concentrated in 

 scattered wart -like batteries ; in the latter case in the head. 

 The coelenteron of the hydroid is sometimes nearly divided into 

 two by a constriction (Tubularia). Lobes (Tubularia) or villi 

 (Myriothela) may project into it from the body wall. 



A hydrorhiza (Figs. 16 to 20) is generally more or less 

 developed in the fixed forms, whether single (Myriothela) or 



