234 



CXELENTEBATA. 



hydrothecre (fig. ISO n}. A more or less stiff mesoclerm lamella is 

 also developed in the interior of the body wall, between the ectoderm 

 and the endoderm. This serves to support the soft parts of the 

 animal, and, in the Mediae, is in part represented by the gelatinous 

 connective tissue of the disc. 



The Medusa (fig. 180 b) is without doubt morphologically higher 

 than the Polyp, since it represents the mature sexual individual, 

 while the Polyp performs the nutritive and vegetative functions. 

 The Medusa, in correspondence with its power of free locomotion, 

 possesses an ectodermal nervous system and sense organs. The 

 nervous system consists of nerve fibres and ganglion cells, and is 

 usually specially concentrated round the edge of the disc, where it 



forms a double ring of 

 fibres running parallel 

 to the circular vessel. 

 The sense organs are the 

 so-called marginal bodies. 

 The generative pro- 

 ducts of the Medusae 

 either have their origin 

 in the ectoderm, in 

 which case they may be 

 developed on the under 

 surface of the disc (sub- 

 umbrella) in the ecto- 

 derm immediately un- 

 derlying the radial 

 canals (Eiicopidce), or in 

 the ectoderm of the 

 manubrium (Oceanidoe) ; or they may arise from the endoderm of 

 the under surface of the umbrella (ticypho medusas). 



Both Polyps and Medusae frequently remain at a lower grade of 

 morphological differentiation, the former becoming polypoid appen- 

 dages, the latter medusoid buds enclosing the generative products. 

 In either case they are situated on the stem or on some part of the 

 Polyp. The individuality of such appendages appears limited ; the 

 medusoid or polypoid animal sinks, physiologically speaking, to the 

 value of a portion of the body or of an organ, while the entire stock 



L. Agassiz, " Contribution to the Natural History of the United State?, Aca- 

 lephae," vol. iii., I860, vol. iv., 18t)2. K Haeckel, -'System der Metluscn," 

 Tom. I. and IT.. Jena, 1880 and 1881. 



FIG. ISO J. Free Medusa of Obelia gelntinosa, as yet 

 without generative organs; g, auditory vesicles. 



