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 rirst 



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 iipper and lower walls along four, six, or 



eight radiating areas, become divided into the vascular pouches 



(gastric pouches), or, as they are called, radial canals, 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 mami- 



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 



fusion 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 subumbrella. 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 (mesoclermic), which sometimes contains cells; while that 



of the oral surface keeps the character of a thin but tirm 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 



