SKELETON" OF CCELENTEEATA. 



107 



Hydroid-Polyps, in wliicli a homogeneous lamella appears between 

 the ectoderm and endoderm; this functions as a supporting 

 lamella to the softer tissues, which are attached to it. Owing to 

 the formation of external tests, this structure has less importance as 

 an organ of support in certain parts of the Hydroida : it is very 

 thin in the parts where the tests are found, but it is much stronger 

 in the free parts of the body, which are not sheltered in the test. 

 Wo find a strong layer of supporting tissue in the wall of the body 

 of the Tubularia, which belongs to the free portion or hydranth 

 of the animal. This tissue consists of a homogeneous substance 

 traversed by fibres, and embedded between the ectoderm and 

 endoderm. This appears to be the first step in the formation of 

 an arrangement, which is highly developed in the Medusa?, the 

 so-called gelatinous disc, but in many of them (Medusas of 

 Clavatella, and Eleutheria) is also only slightly developed. 



The gelatinous disc is in the Hydromedusas either completely 

 homogeneous or is traversed by fibres, which extend from the 

 ectoderm to the endoderm. It forms a disc, which is attached to 

 the aboral surface of the body, and which determines the form of 

 the body (Fig. 39, /) ; it 

 may become modified into 

 the form of a bell. The 

 organs derived from the 

 endoderm, which consist 

 principally of the gastric 

 apparatus, lie on the oral 

 surface of the disc. 



Although the gela- 

 tinous umbrella of the 

 Discophora agrees in its 

 external characters with 

 that of the Hydrome- 

 dusas, it differs from it in 

 some not unimportant 

 characters. Its substance 

 contains various morpho- 

 logical elements, which resemble those of gelatinous connective tissue, 

 and it is continued on the oral surface over the so-called stomachal 

 stalk. It surrounds, therefore, the greater part of the gastrovascular 

 system. 



Of less moment are the characters presented by the supporting 

 structures of the tentacles of many Hydromedusas. In both the 

 Hydriformes and Medusas (Trachynemidas, .ZEginidas) the axis of 

 the tentacle is formed of a series of cells, the elements of which 

 appear to be encapsuled by a more or less homogeneous mem- 

 branous layer (cf. Fig. 9). The rows of cells are thus to a certain 

 extent rigid. A ring of much the same structure (annular cartilage) 

 is found on the edge of the disc of the Geryonidte. 



Fig. 39. Diagram of a vertical section through a 

 growing Cunina rhododactyla ; on the right side 

 through a radial, on the left through an interradial 

 vertical plane, b Marginal vesicle, c Radial canal. 

 g Generative products, h Mantlefold. h Stomach. 

 I Gelatinous disc, r Radial pouch, t t Tentacle. 

 t w Base of tentacle, v Velum (after E. Hiickel). 



