Scyphozoa

SCYPHOZOA, a group of jellyfish belonging to that series of animals known as the Coelenterata (q.v.) whose general char acteristics are described in a separate article, other groups of jellyfish being dealt with in the articles HYDROZOA and CTENO PHORA. The scyphozoan jellyfish differ from all others in their anatomical characters and, speaking generally, they are larger and more substantial than hydrozoan medusae. Some of them attain extremely large size, measuring as much as 7ft. across the bell; these are the largest known coelenterates.

From the point of

view of the general study of the Coelenterata the main interest of the Scyphozoa lies in their life-history. The egg sometimes develops directly into a medusa, but in other cases there is a complicated life-history, which may be exemplified by the case of the Aurelia (figs. I and 2). Aurelia is one of the jelly fish most commonly stranded on British shores, and is a trans parent medusa usually from 3 to 6in. across in these waters. It

has a rather shallow bell tinged with mauve, becoming a darker colour at the sex-glands, which show through from the inside.

It swims by a rhythmic series of contractions of the bell. The fertilized egg of Aurelia develops, not into a miniature medusa, but into a polyp of distinctive structure known as a scyphistoma —a small trumpet-shaped creature with long marginal tentacles, dition by degrees from this point onwards. Occasionally a seg ment of a strobila becomes a polyp instead of a medusa.

Such a life-history provides an interesting example of that type of polymorphism known as alternation of generations. (See COE LENTERATA and HYDROZOA.) The permanent polyp-generation alternates regularly with a relatively transient medusa-generation, and the medusae alone are sexual and produce ova and sperma which attaches itself by its aboral end to a foreign support (see COELENTERATA where its feeding mechanism is also described, and