IV PHYLUM CCELENTERATA 143 



zooids. The hydranths may be colourless and quite invisible to 

 the naked eye, or, as in some TubulariiE (Fig. 105, G) may be bril- 

 liantly coloured, flower-like structures, nearly an inch in diameter. 

 The medusEe may be only just visible to the naked eye, or, as in 

 Aiquorca, may attain a diameter of 38 mm., or about 15 inches: 

 they are often seen with great difficulty owing to the bubble-like 

 transparency of the umbrella ; but frequentl}- the manubrium is 

 brightly coloured, or brilliant dots of colour — the ocelli or eye-spots 

 — may occur around the margin of the umbrella. They are also 

 frequently phosphorescent, the phosphoi'escence of the ocean being 

 often due to whole fleets of medusiE liberated in thousands from 

 the hydroid colonies beneath the surface. 



The two sub-orders of Leptolinse are distinguished by the 

 arrangement of the perisarc. In the Anthomedusae, of which 

 BoiKjainvillea (Fig. 104) is a good example, the cuticle stops short at 

 the bases of the hydranths, and the reproductive zooids are not 

 enclosed in gonothecse. It is for this reason that, in classifications 

 founded on the zoophyte stage, the Anthomedusae are called Gymno- 

 blastea or naked-budded zoophytes (see also Fig. 105, 1, 4-, 5). In 

 the Leptomedusse the cuticle is usually of a firmer consistency than 

 in the first sub-order, and furnishes hydrothecse for the hydranths 

 and gonothecse for the reproductive zooids : they are hence often 

 classified as Oalyptoblastea or covered-budded hydroids. To this 

 group belong the commonest species of hydroids found on the sea- 

 shore, and often mistaken for seaweeds — the " Sea-firs " or Sertu- 

 larians. 



The medusae also exhibit characteristic differences in the two 

 sub-orders. In the Anthomedusae the umbrella is usually strongly 

 arched, and may even be conical or mitre-shaped (Figs. 104 ; 105, 7 ; 

 1 09, 1 and 2) : its walls are thick, owing to a great development of 

 the gelatinous mesogloeaof the ex-umbrella, that of the sub-umbrella 

 remaining thin ; and the velum is considerably wider than in Obelia. 

 But the most important characteristics are the facts that the 

 gona,ds{gon) are developed on the manubrium and that lithocysts are 

 absent. Sense-organs are, however, present in the form, of specks 

 of red or black pigment at the bases of the tentacles. These ocelli 

 (oc) consist of groups of ectoderm cells containing pigment, and it 

 has been proved experimentally that they are sensitive to light : 

 they are, in fact, the simplest form of eyes. In the Leptomedusse 

 the umbrella is usually less convex, thinner, and of softer consist- 

 ency than in the Anthomedusae, the gonads are developed as buds 

 formed in connection with the radial canals and projecting from 

 the sub-umbrella, the velum is feebly developed, and sense-organs 

 take the form sometimes of ocelli, but usually of lithocysts. 



In the majority of Leptolinse the coenosarc, as in Obelia, con- 

 sists of a more or less branched structure attached to stones, timber, 

 seaweeds, shells, &c., by a definite root-like portion (hydrorhiza). The 



