MICROGRAPHIC DICTIONARY. 



. Aclass in the Animal King- 

 dom, commonly known as Sea-nettles, on 

 account of their producing a sensation of 

 indication when touched, or Jelly-fishes, from 

 their gelatinous consistence. 



They are transparent, floating and free, 

 discoid or spheroid, and vary in size from 

 an almost invisible speck to a yard in dia- 

 meter. Then* organs are arranged in a ra- 

 diate manner around a central point and lon- 

 gitudinal axis, which are occupied by the 

 digestive apparatus. The disposition of the 

 parts is generally quaternary. 



The body is usually composed of a trans- 

 parent gelatinous substance, closely resem- 

 bling the vitreous humour of the eye in the 

 Vertebrata, and consisting of delicate and 

 transparent polyhedral cells of various sizes 

 filled with colourless liquid ; these are some- 

 times nucleated, at others not. 



The cutaneous surface of the body is co- 

 vered with a very delicate epidermis (PL 40. 

 fig. 2). Cilia exist on various parts of the 

 body, especially the arms, tentacles, cirrhi, 

 &c. ; upon which also peculiar stinging organs 

 and organs of adhesion occur. In those spe- 

 cies which are notorious for their stinging 

 powers, these stinging organs are also situ- 

 ated in aggregations beneath the epidermis 

 of the body. The stinging organs usually 

 form oval capsules, in which a spirally coiled 

 filament is enclosed (PL 40. fig. 3 a, b) ; 

 this flies out on the slightest touch, with the 

 capsule to which it is attached, from the 

 irritated part of the skin (PL 40. fig. 3c,d). 

 In some Acalephse, these stinging organs are 

 replaced by oval capsules from which a rigid 

 bristle projects (PL 40. fig. 4). These do 

 not produce urtication, but enable the animal 

 to adhere to other bodies. It would be of 

 great interest to determine whether these 

 stinging organs give indications of the pre- 

 sence of formic acid, as they much resemble 



in appearance similar organs occurring in 

 Hydra, in which we have met with such in- 

 dications. Near the surface of the body and 

 between the cells composing its substance, 

 pigment-cells frequently occur, some of 

 which are isolated, others aggregated into 

 groups. The paler and more delicate co- 

 lours are said to arise in some instances 

 from pigment uniformly dissolved in the sub- 

 stance of the body; it is most probable, 

 however, that they arise from iridescence. 



A distinct muscular system is present, in 

 the form of long, thin, reticular muscular 

 fibres and bundles, almost everywhere per- 

 vading the contractile substance of the body. 

 These muscles appear to be transversely stri- 

 ated. 



The floating and locomotion of these ani- 

 mals is aided by larger or smaller cavities 

 filled with air. 



The nervous system consists of a ring with 

 eight ganglia surrounding the oesophagus, 

 with delicate filaments issuing from them, 

 and a single ganglion at the opposite end of 

 the body; and in the Medusae there are 

 ganglia at the bases of the tentacles. 



The organs of sense consist of tubercular 

 or spathulate bodies situated at the margin 

 and end of the body, and connected with ad- 

 joining ganglia. These were regarded as 

 organs of vision ; and consist essentially of a 

 membranous capsule containing a clear liquid 

 with crystals of carbonate of lime, and some- 

 times a red pigment (PL 40. fig. 5). But 

 as many of them contain no pigment, they 

 have been considered as of auditory function, 

 and the crystalline bodies as otolithes. 



The digestive cavity is lined with ciliated 

 epithelium and furnished with distinct walls, 

 which are directly continuous with the gene- 

 ral parenchyma of the body, so that there is 

 no abdominal cavity. The mouth is either 

 single and central, or multiple. In the former 



