THE COELENTERATA 167 



of nervous control. Mesoglea is excessively developed, not so much 

 for protection as to lower the specific gravity of the animal to allow 

 of floating. The hydranth, on the other hand, is a fixed form not 

 needing locomotor, sensory and nervous specialisation, but requiring 

 a longer, stalk-like body to hold it up from the ground and a thinner 

 tougher mesoglea for support. The exumbrella of the medusa, 

 which is the part attached to the blastostyle, corresponds to the foot 

 of the hydranth, while tentacles, mouth and manubrium are similar 

 structures in both forms. 



When living the medusa swims slowly by a series of sudden 

 contractions of the umbrella, that propel it through the water aboral 

 side foremost, by forcing out the water contained in the sub- 

 umbrella space. It is not capable of active locomotion, being in the 

 main a floating form. The power of contraction lies partly in the 

 epithelio-muscular cells of the sub-umbrella ectoderm, but their 

 efficiency is greatly augmented by a definite ring of muscle fibre cells 

 around the margin of the umbrella. These cells are ectoderm cells 

 that have become much modified and then: muscle processes greatly 

 enlarged, until ultimately they form almost pure muscle fibres, 

 which sink down into the mesoglea, and so come to take up a sub- 

 epidermal position. Here then we have another advance on Hydra, 

 the production of a specially developed ring of muscle elements. 

 The food is caught by the tentacles and conveyed to the mouth as 

 in the polyp, thence it is passed to the stomach and digested. The 

 substances in solution are circulated to all parts by the gastro- 

 vascular canals, and the undigested residue is expelled through the 

 mouth, no anus being developed. 



The sexual reproduction in Obelia is brought about by these 

 medusoid persons, which are the bearers of the germ cells. They 

 are dioecious, that is to say, the sexes are separate, and any individual 

 medusa is either male or female, but not both, the four gonads in 

 the one case being testes, and in the other ovaries. When first 

 liberated the medusae show no signs of the sexual cells, and even 

 when the downgrowths of the radial canals, the future gonads, 

 hpve been formed there is still no sign of the germ cells. It is 

 interesting to note that the oogonia and spermatogonia do not make 

 their first appearance in these gonadial rudiments, but on the other 

 hand they are first discernible in the walls of the manubrium. 

 Thence they migrate along the entoderm of the gastral cavity and 

 radial canals to their definitive position between the ectoderm and 

 mesoglea, where they undergo maturation. It is not until this 

 migration has taken place that we can strictly term the down- 

 growths gonads. In other allied forms the primordial germ cells 

 first appear in the hydrocaulus, and then wander to the place where 



