184 



ZOOLOGY 



SECT. 



In many of the Discomedusae development takes place in the same general 

 way as in Aurelia, i.e. the impregnated egg gives rise to a scyphula or asexual 

 polype stage, which, by transverse division, pi-oduces sexual medusae. In 

 Cassiopeia the scyphula arising from the fertilised ovum gives off buds which 

 become detached as free-swimming planulse, and these, coming to rest, develop 

 into scyphulse. But in other cases there is no alternation of generations, and 

 development is direct. For instance, in Pelagia (Fig. 136) — one of the 

 Semostom;e — a blastula is formed which becomes invaginated at one end, 



Fig. 136.— Pelagia noctiluca : Three developmental stages, m. mouth; r. marginal lappet : 

 ». tentaculocyst. (Prom Korschelt and Heider, after Krohn.) 



forming a gastrula. The blastopore or gastrula-mouth remains open, and a 

 considerable space is left between the invaginated endoderm and the ectoderm. 

 Next the mouth region becomes elevated, forming a manubrium, and around 

 this a circular depression appears — the rudiment of the sub-umbrellar cavity — 

 surrounded by a raised ridge, the umbrella margin, which soon becomes divided 

 into lobes, the marginal lappets. Up to this time the embryo is ciliated 

 externally, but soon the cilia disappear, and the little creatures assume somewhat 

 the form of an ephyrula, which gradually develops into the adult Pelagia. 



Additional Remarks on the Scyphozoa. 



The Scyphozoa are all marine, and the majority are pelagic, i.e. 

 swim freely on the surface of the ocean. A few inhabit the deep 

 sea, and have been dredged from as great a depth as 2,000 fathoms. 

 Nearly all are free-swimming in the adult state : some, however, 

 live on coral-reefs or mud-banks, and are found resting, in an 

 inverted position, on the ex-umbrella ; and a few, such as Lucern- 

 aria, are able to attach themselves at will by a definite ex- 

 umbrellar peduncle. 



Many of' the Scyphozoa are semi-transparent and glassy, but 

 often with brilliantly coloured gonads, tentacles, or radial canals. 

 In many cases the umbrella, oral arms, &c, are highly coloured, 

 and some species, e.g. Pelagia noctiluca, are phosphorescent. They 

 are all carnivorous, and although mostly living upon small 



