iv PHYLUM CCELENTERATA 183 



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 lappet?. Up to this time the embryo is ciliated 

 externally, but soon the cilia disappear, and the little creatures assume some- 

 what 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 

 organisms, are able, in the case of the larger species, to capture 

 and digest Crustaceans and Fishes of considerable size. In many 

 cases small fishes accompany the larger forms and take shelter 

 under the umbrella. 



Considering the extremely perishable nature of these organisms, 

 and the fact that many of them contain not more than 1 per cent. 

 of solid matter, it is not to be expected that many of them should 

 have left traces of their existence in the fossil state. Nevertheless, 

 in the finely grained limestone of Solenhofen, in Bavaria, belong- 

 ing to the Upper Jurassic period, remarkably perfect impressions 

 of Jelly-fishes have been found, some of them readily recognisable 

 as Discomedusse. 



CLASS III, ACTINOZOA. 



1. EXAMPLE OF THE CLASS. A SEA-ANEMONE 

 (Tealia crassicornis). 



Sea-anemones are amongst the most abundant and best known 

 of shore-animals. They are found attached to rocks, sea- weeds, 

 shells, &c., either in rock-pools or on rocks left high and dry by the 

 ebbing tide. Usually their flower-like form and brilliant colour 

 make them very conspicuous objects, but many kinds cover them- 

 selves more or less completely with sand and stones, and contract 

 so much when left uncovered by water, that they appear like soft 



