Class 1. Rydrozoa. Order 3. Acalepha. 



Ill 



The Acalepha^ several of wHicli are phosphorescent, occur only 

 in the sea. The following may be cited as examples : — 



1. The Oommon Jelly-fish (Aurelia aurita), with disc only slightly convex, 

 bearing at its margin numerous short tentacles; the marginal bodies include 

 both auditoiy and optic organs (tentaculooyst). The enteric cavity is produced 

 into four short radial sacs, containing the four gonads, each shaped like the edge 

 .of a human ear. The mouth is provided with four large oral tentacles. Like 

 its alhes, this animal contains a large amount of water (95 — 96 per cent, 

 water, 4 — 5 per cent, dry substance). Very abundant in N. European seas. 



2. Cyanea capillata, a large and beautiful medusa, which is specially 

 characterised by the arrangement of the extraordinarily long marginal tenta- 

 cles in eight groups, on the under surface of the much lobed disc. The 

 -nematocysts cause an intensely burning sensation in thin-skinned portions of 

 •the human body. Commonly found with Aureha. 



Class 2. Anthozoa. 



The body is cylindrical, and contains a large internal digestive 

 cavity, into which the upper region of the body-wall is invagi- 

 nated to form the stomodffium, as 

 .already explained. The external 

 jDpening of the stomodasum is 

 -usually termed the mouth, but it 

 must be remembered that the 

 true mouth, the entrance into the 

 digestive cavity (homologous, with 

 the mouth of a medusa) is 

 situated at the lower end of the 

 stomadeeum. The former may be 

 termed the external mouth, 

 the latter, the internal mouth. 

 Within the enteric cavity there are 

 -perpendicular radial mesenteries, 

 reaching from the stomodseum to 

 the body- wall above,* whilst below, 

 the inner edge is free (something 

 like the dissepiments of a poppy- 

 head). The number of mesen- 

 teries varies (8, 12, etc.). At the 

 upper end of the animal there 

 is a circle of tentacles (in 

 exceptional cases several circles) 

 the number corresponding with that 



Fig. 6G. Longitudmal section through 

 a. solitary Anthozoon, diagrammatic ; the 

 section passes through a mesentery on the 

 right side, but hetween two on the left. 

 i endoderm, mi mesoglEea, y ectoderm, 

 m internal, m' external month, s stomo- 

 dseum, t tentacle. — Orig. 



* In most Actinia, which possess a large number of mesenteries, only some 

 reach from the body- wall to the stomodseum, in the others, the inner border is free 

 for its whole length. 



