THE TRACHOMEDUSjE. 



291 



The 



planulse settle down and elongate, a sheath is formed around them and tentacles arise. By- 

 and-by the stem branches, and a colony is established, the gonosome stems producing the 

 medusae in due time. 



The Equoridse are the last family, and the genus Zygodactyla (twin finger) is its type, 

 medusae of this genus are from seven to eight inches in diameter and 

 light violet in colour. The tentacles are long and fibrous, and dark violet, 

 and can be contracted to a mere fringe. They often remain motionless 

 in the water. It is an inhabitant of the North Atlantic and Northern 

 Seas generally. 



In the genus Eucope the tentacles are well developed, and the calycular 

 membrane which comes up to their bases diminishes in size until it joins 



the ringed tissue of the stem. On one side 

 of this a long and rather fusiform projection 

 exists, and it has a central body, around 

 which medusa buds form. This gonophore 

 permits the medusaB to escape. They are 

 flat, with a little knob at first on the top, 

 and a few short tentacles are around the 

 disc. Really the knob is the proboscis of 

 the mouth, and the little medusa turns inside 

 out with ease. MARGIN OF A MEDUSOID. 



a, circular canal; 6, ocellus; c, sae 

 or lithocyst; c, spherule of lime; 

 d, tentacle. (A/ter 



EUCOPE DTAPHANA. 



A, the colony ; B, the young medusae within 

 the reproductive part. (After L. Agawiz.) 



SUB-ORDER TRACHOMEDUS^E. 



The Trachomedusse have a gelatinous 



disc, which feels decidedly hard to the fingers, and the margin is 

 usually lobed. The tentacles are either rigid or else can move, 

 and there are peculiar sense organs on the base of the tentacles, 



accompanied by lithocysts, and sometimes by eye-spots. They do not pass through any colonial 

 stage, and the eggs develop into a ciliated larva formed of two layers of tissue, and it has no 

 stomach, but becomes elongated into two arms. After a while two other arms or tentacles are seen, 

 and the central cavity and mouth. Reproductive organs appear, and then more tentacles. There 

 are numbers of these Trachomedusae in the oceans, and many genera have been distinguished. 



In the fresh warm water (86 Fahr.) of the tank which contains the Victoria Regia in the Botanic 

 Gardens of the Regent's Park, London, Mr. Sowerby found great numbers of minute medusas moving 

 with great vivacity, and preying on the minute Crustacea. They were about a line to half an inch 

 in diameter, and had nearly 200 tentacles and four radiating canals and a circular one. There 

 was a velum, and the margin had many eye-spots. The manubrium is long and expanded below, 

 and the tentacles are solid. The genus has been called by Dr. Allman Limnocodium, and Prof. Ray 

 Lankester believes that probably it is one of the Trachomedusae. It is probably the only instance 

 of a medusa which can live in perfectly fresh water, and which dies in cold or salt water. Nothing 

 is known about its origin. 



There are many families of this sub-order, and four may be especially noticed. The Trachonemkta 

 have the marginal filaments rigid and hardly movable, and the sexual organs develop in vesicular 

 swellings in the eight radial canals, and some have a flat disc with club-shaped tentacles. 



Some, like the ^Egiua, with rigid marginal tentacles, belong to the family ^Eginidae. The 

 stomach-pouches reach far towards the edge of the disc, and the sexual elements are produced by the 

 derm of their sides. Sometimes there is no marginal canal, and four tentacles often exist. 



The Geryonidse have a large cylindrical peduncle environing the stomach. Four or six canals 

 are in the umbrella, and extend from the bottom of the stomach to the radial canals in which the 

 reproductive organs exist. Finally, the Charybdeidae have the borders of the umbrella with tentacles 

 and compound marginal corpuscles. Ramified canals come from the processes of the stomach. They 

 are dwellers in the Mediterranean and Atlantic. 



