COELENTERATA. 119 



absent ; sexual cells arise in the ectoderm of the manubrium, or along 

 the radial canals ; they are rarely developed directly from the egg. 



The Acraspeda, on the other hand, have a lobed umbrella edge and 

 no double nerve ring; the marginal sense bodies are always tentacular 

 in form, and contain a prolongation of the gastrovascular system ; 

 gastral filaments are always developed on the stomach wall, and 

 taenioles are generally present ; the sexual cells are endodermal, and 

 the development is either direct or by fission from a Scypho-polyp. 



Connection between the Polyp and the Medusa. The medusa 

 is very generally produced by budding from a polyp, although in 

 certain orders it does rise directly from the egg. For a long time 

 it was considered a remarkable circumstance, hardly admitting of 

 a satisfactory explanation, that organisms which differed so widely 

 as polyps and medusae they had indeed been systematically separated 

 in different classes should only form different stages in the life- 

 history of a single cycle of development. The theory of "Alternation 

 of Generations" contained only a description of the matter, and 

 offered no explanation. The discovery of the mode of origin of 

 the medusa as a bud on the body of the polyp first clearly demon- 

 strated the direct relation of the two forms ; for it proved that the 

 medusa is a flattened disc-shaped polyp with a shallow, but wide 

 gastric cavity, the peripheral part of which has, by the fusion (Fig. 

 107, c) of its upper and lower walls along four, six, or more radiating 

 areas, become divided into the vascular pouches, or as they are often 

 called, radial canals. The differences consist mainly in the presence 

 of the oral cone or hypostome of the polyp as a well-marked external 

 appendage, the manubrium ; in the enlargement of the oral disc to 

 form the muscular sub-umbrella surface (Fig. 107, c, S) ; in the 

 collapse of the coelenteron (leaving the endoderm lamella) above 

 referred to; and in the greater development of the nervous and 

 muscular systems in connection with the free -swimming habit of 

 life. To these may be added the thickening of the structureless 

 lamella between the dorsal endoderm and ectoderm, to form the jelly- 

 like tissue which is so characteristic of medusae. 



As stated above the medusa is generally produced by budding from a polyp- 

 colony, but however produced it always develops the generative cells. For this 

 reason it is frequently called a gonophore. This name, though not employed when 

 the polyp stage is unimportant (Acalephac) or absent (some Acalephae, Tracho- 

 medusae, etc.), is applied to all medusae or medusa-like organisms which are 

 produced by budding from a polyp. Typically a gonophore becomes detached 

 from its origin, and as a free -swimming medusa distributes the generative 

 products far and wide ; but in some colonies the gonophore does not become 



