90 LOWER INVERTEBRATES. 



Probably all are marine and many genera gregarious, either seeking each other's 

 company, or huddled together by ocean currents or tide eddies. 



At spawning-time they are said to don brighter colors, or at least their ovaries and 

 spermaries at that time become more highly colored. Like the hydroids, their 

 organs for defence and offence are the stinging-cells by which their bodies are covered. 

 Many sting with great violence, while others can be handled with impunity. None, 

 however, are destitute of stinging-cells. 



All are phosphorescent, especially when irritated, while the color and intensity 

 of the emitted light varies with the genus. 



One of the most common Discophores in New England waters is called Cyanea, and 

 is a representative of a family of moderate size known as the Cyaned j;. The most 

 striking peculiarity of Cyanea is its disk-shaped body, which varies in size from that 

 of a penny to several feet in diameter. Its color is reddish brown, but when dead and 

 washed about for some time it becomes light blue. The body-disk is divided into two 

 well-marked regions, called the aboral and the oral, or the upper and lower surfaces 

 as the animal naturally swims in the water. The upper surface is smooth and without 

 appendages, save little filaments which are remnants of bodies of considerable 

 size in the young of the animal. The under or oral surface is so called from the fact 

 that from it hangs not only the stomach with its mouth, but also many other import.ant 

 structures. Tlie thickness of the disk in its centre is much greater than at the 

 pcripherj', where it becomes very thin and flexible, and capable of considerable motion. 

 Around the margin of the bell are found at regular intervals eight sense-bodies, which 

 lie in deep incisions in the rim. Each sense-body is a small sac or cyst mounted on a 

 short peduncle, and in the interior there are a number of rhombohedral otoliths of 

 calcareous composition. Each sense-body is covered by a thin, gelatinous wall stretched 

 above it, which is known as the " hood." From the existence of this hood in the 

 Discophora, and its absence in the Hydroidea, Siphonophora, and a few other jelly- 

 fishes, those animals are called the hooded-e}ed medusa?, while the latter are sometimes, 

 especially in older writings, designated the naked-eyed jelly-fishes. 



The most prominent of the several appendages which hang from the oral surface 

 of the disk is a thin, curtain-like body of great breadth, which is thrown into a great 

 number of folds and frills. This curtain is open beloAV, and its inner walls make the 

 walls of the stomach. It hangs far down below the oral surface of the bell, extending 

 far beyond it as the medusa, by strokes of the margin of the disk, is driven along 

 tlirough the water. 



The tentacles of Cyanea are found in bundles, in each of which there is a oreat 

 number of these organs. Each tentacle is long, thread-like, and very contractile, 



possessing stinging cells which, however, 

 are rather feeble in their action in the genus 

 Cyanea. The tentacles in larger specimens 

 of the genus reacli an extraordinary lengtl), 

 and, as in other Discophora, have for their 

 function the capture of the food. 



The genus Aurelia, the type of the 

 family AuRELiiD^E, is, next to Cyanea, one 

 of the most common representatives of 

 the Discophora in New England waters. Although it never reaches the great size 

 attained by the former, it may well be ranked as one of the largest of our Acalephs. 



Fig. 84. — Aurelia JIacidula. 



