28 ZOOLOGY. 



two feet, and the weight of large ones readies fifty or sixty pounds. Yet 

 this great mass is composed almost entirely of water, which pervades the 

 tissues, and these are of such extreme tenuity that the weight of one of 

 these masses is reduced by desiccation to grains instead of pounds. 



The more familiar forms belong to the Pulmonigrada, also termed Disco- 

 phora, whicli may be compared to an expanded umbrella, or to a mushroom, 

 the alternate contraction and expansion of which enable the body to move 

 through the water with the convex or upper surface foremost ; a mode of 

 progression which has afforded a name to the order, from its resemblance 

 to the action of lungs. 



Tlie beauty of many of these animals equals anything in organic nature ; 

 the colors are prismatic or entirely wanting, and in the latter case, the 

 gelatinous transparent body resembles a mass of colorless liquid gum, which 

 can only be distinguished by its motions from the water which surrounds it. 



•' When in a jar or basin they are often very difficult to distinguish, but 

 by placing the vessel in the sun, we see their shadows floating over the sides 

 and bottom like the shadows of Hitting clouds on a landscape. These soon 

 guide us to the creatures themselves, and before long we distinguish their 

 ocelli and colored reproductive organs." — Forbes. 



The disk forming the greater part of the body varies from hemispherical 

 to flattened discoidal, and is sometimes lengthened into a conical or sub- 

 cylindrical form. The central portion is thickest, and the inferior surface 

 is concave. The margin is either entire or fringed with tentacles, which 

 vary greatly in length, number, and form. Some of these tentacles have a 

 colored spot at their base called an ocellus, and upon this Forbes has divided 

 the Discophora into two groups ; namely, the Steganojjkthalmata (covered 

 eyes), in which the ocelli are protected by membranous lobes, and the 

 Gymnopldhalmata (naked eyes), in which the ocelli are not protected. The 

 former are more highly organized than the latter, and in most of the genera 

 the sexes are not united in the same individual. Agassiz has discovered a 

 nervous ring aroimd the mouth, with branches extending to the ocelli ; an 

 arrangement which resembles that in the Echinodermata. Ehrenberg had 

 made a less distinct announcement, and Dr. Grant announced the discovery 

 of a nervous system in Beroe, in the year 1833. 



From the centre of the concavity of the disk arises the peduncle, which 

 varies much in size and shape, in some genera forming a considerable 

 portion of the animal, and in others being reduced to a slender extensile and 

 contractile tube, at the extremity of which the mouth is situated. The 

 cavity of the peduncle, or its base, is the stomach, whence branches are sent 

 towards the disk, around the margin of which there is a canal connecting 

 with them. These radiating gas tro- vascular branches vary in number 

 from four to twelve or more. In the naked-eyed genera they are seldom 

 branched ; and when they are, the branches run to the marginal canal, as 

 in the genus Wilsia* (Forbes, Monog. of the British Naked-eyed Medusae 



* Named after Dr. Will, who wrote on this subject. 

 TAI 



