THE HYDROMEDUSAE 



medusoid develop into a hydroid. In other cases the medusoid 

 may develop directly from an egg-cell, or may lie budded from 

 another medusoid. Xo gastric ridges or filaments occur in 

 either hydroid or medusoid. The sexual cells lie typically on 

 radii of the first order, and are always (?) primarily derived from 

 ectoderm cells. The medusoids are characterised by the possession 

 of a muscular non-vascular velum, and have as sense organs ocelli, 

 otocysts, or tentaculocysts. 



The Diblastula and the Embryonic Layers. — The single 

 form of Hydromedusan cell, which was excepted above as being 

 capable of independent existence, is called the egg or ovum. If duly 

 fertilised the ovum shortly splits into two cells, which in their 

 turn divide again ; this process of division, or segmentation of the 

 ovum, is contiiuied until ultimately, by one path or another, an 

 embryo has been built up which consists of numerous cells, arranged 

 in two layers round a central cavity. To an embryo of this kind 

 the name diblastula (gastrula) has l)een given (Fig. 2). These 

 two layers of cells, however complex may be the ultimate form 

 of the adult organism, are the chief constituent tissues of all 

 Hydromedusae, as was shown by Huxley so long ago as 1849. 



To the outer layer or skin has 

 been assigned the name ecto- 

 derm; the inner layer which 

 lines the central cavity or 

 roelenteron has been termed the 

 endoderm. Between the ecto- 

 derm and the endoderm is 

 deposited later a gelatinous 

 secretion, the non-cellular 7neso- 

 ijloea, into which cells from either 

 of the two primary layers may 

 wander. From these simple 

 elements — ectoderm, meso- 

 gloea, and endoderm lining the 

 coelenteron — all the varied and 



FlO. 2. 



Fig. 1. 



1. —Section tlirou;^)) a blastula ; the single layer 

 of cells surrounds a cavity, the blastocoele. At 

 thu lower \io\t two cells of the future endoderm 

 have been budded into the blastocoele. 



2.— Section through a diblastula (gastrula). 

 The cells of the future ectoderm are ciliated ; 

 by their proliferation a number of cells, the future 

 endoflerm, have l)een budded into the bla.stocoele, l)eautiful fomiS of the HvdrO- 



medusae are moulded. 



which they nearly HU. 



General Description of the Hydroid and of the Medusoid. 

 — In no group of the animal kingdom is polymorphism carried to a 

 greater extent than in the Hydromedusae, yet, upon morphological 

 analysis, the numerous forms which individuals exhibit arc 

 apparently all referable to modifications of one or other of two 

 main types — the Hydroid and the Medusoid. 



The Hydroid (hydriform jierson, hydranth, trophozooid) is 

 represented in a simjile form by the genus Hydra, from which it 

 derives its name. This presents (Figs. 3, 4, B and C) a tubular 



