Cwlentera. 99 



■consisting in the presence of a gelatinous layer between the inner 

 and outer layers of cells, the two latter corresponding with the 

 hypoblast and epiblast of the gastrula. In addition to this, however, 

 as will be seen more fully later on, the cells of each layer are 

 not all similar as in the gastrula, but have been developed in 

 different ways, some as muscle cells, others as nerve cells, etc. 



Allied to this simpler form, and capable of being derived from it 

 are other types of somewhat more complicated structure. Very 

 often (Pig. 58, B) the lower closed end of the sac is broadened out to 

 form a convex disc, so that the animal resembles an old-fashioned 

 candlestick. The discoid portion consists of the same layers as the 

 rest of the body ; the mesoglsea is, however, especially well developed 

 on the convex side, and the two lamiuEe of the endoderm, which cover 

 respectively the upper and the lower walls of the cavity in this 

 portion, are fused in certain regions, so that instead of a flat simple 

 cavity there is a regular system of canals ; at the lines of fusion 

 the endoderm shrinks to a thin membrane. Whilst Coelentera of 

 the first and simplest type are usually sessile, those just described, the 

 medusoid forms, are generally free-swimming ; the disc is turned 

 upwards in swimming, the opening of the sac downwards. 



A third type occurs in the coral polyps (Fig. 58, C) . 

 Here the cavity of the sac is very wide, and its upper portion is 

 invaginated, so that the true mouth is situated at the base of the 

 turned in portion or stomodeeum. 



The Coelentera are usually provided with soft appendages, the 

 tentacles, which are outgrowths or evaginations of the body-wall and 

 consist of the same layers ; they are usually found near the mouth, 

 but in the medusae are carried out on to the edge of the disc. 



In the whole structure of the body, e.g., in the arrangement of the 

 appendages just mentioned, and in the distribution of the canals in 

 the disc of medusse, a more or less sharply marked radial 

 symmetry is evinced, such that the chief axis, about which the 

 radial structures are arranged, corresponds with the central axis of 

 the sac. The number of rays differs in the various Ccelenterate 

 groups; most frequently the body is divided into four, or a multiple 

 ■of four rays, in others into six, or a multiple of six. 



The ectoderm, which forms the outer covering of the body 

 (analogous with the epidermis of Metazoa), is a peculiar epithelial 

 layer, the cells of which are developed in very various ways. 

 Some are simple epithelial cells, either flat or cylindrical, and pro- 

 vided with one or more cilia; such cells are quite comparable with 

 the epidermal cells of other Metazoa, and on this account they are 

 .termed true epidermal cells. Others, like these and occurring 

 among them, differ in that their inner ends are modified to 

 form contractile fibrils, so that their outer protoplasmic portions 

 perform the function of epithelial cells, their inner portions of muscle 



H 2 



