HYDEOZOA. 235 



approaches more nearly to a single organism. The more completely 

 polymorphism and division of labour are impressed upon the polypoid 

 and medusoid appendages, so much higher becomes the unity of the 

 whole which is morphologically a colony of animals. In these cases 

 it is often difficult to distinguish between budding and simple growth. 

 For a long time it was considered as 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 as different classes should only form dif- 

 ferent stages in the life-history of a single cycle of development and 

 thus be united in the closest genetic connection. 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 demonstrated 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 of its upper and lower walls along four, six, or 

 eiyht radiating areas, become divided into the vascular pouches 

 ((jastric pouches), or, as they are called, radial canah, which 

 correspond to the gastrovascular pouches of the Anthozoa. The 

 differences consist, in connection with the discoidal form, mainly in 

 the position of the gastric tube as an external appendage, the manu- 

 brium, and in the great reduction in height of the radially extended 

 septa (mesenteries), which are traversed by a layer of endoderm cells, 

 the vascular or endoderm lamella. This layer is derived from the 

 fusi9n mentioned above of the aboral with the oral layer of the 

 endoderm of the peripheral part of the gastro-vascular cavity. At 

 the same time the oral disc becomes enlarged and concave to form 

 the cavity of the bell, the ectodermal lining of which gives rise to 

 the muscles of the suburnbrella. The supporting substance of the 

 arched (after it is freed from its attachment) aboral surface of the 

 disc becomes very much thickened and gives rise to the gelatinous 

 substance (mesodermic), which sometimes contains cells ; while that 

 of the oral surface keeps the character of a thin but firm lamella, and 

 serves as a support for the muscles on the under surface of the disc. 

 The tentacles accordingly arise near the edge of the disc, and become 

 the marginal tentacles of the Medusa. In addition to these, four 

 simple or branched oral appendages appear as outgrowths from the 

 manubrium. 



In addition to the sexual reproduction, asexual multiplication is 



