RADIATA. 



695 



CLASS ACALEPHiE. 



The classification of this group proposed by Cuvier was founded upon a very imperfect acquaintance 

 with the animals it includes ; and it is now altogether abandoned. Much remains to be known, how- 

 ever, with respect to the internal structure of many of the tribes with whose external forms we are 

 familiar ; and it is probable that no classification yet proposed will remain without considerable modi- 

 fication from future discoveries. That which is at present most generally received is based on the 

 mode of locomotion peculiar to the different tribes ; according to which the class is divided into the 

 four orders, Pulmograda, Ciliograda, Cirrhigrada, and Physograda. 



I. The order Pclmoorada, or Discophorje, including all the ordinary Medusce, is characterized by the regular 

 discoidal or circular form of the animals composing it. The body is of gelatinous texture, without any in- 

 ternal solid skeleton ; the stomach is placed under the centre of 

 the disk, and usually opens by a single central mouth ; round 

 the stomach are placed the ovaries, opening by separate aper- 

 tures ; the margin of the disk is usually furnished with cirrhi or 

 tendril-like appendages, but these are not extensible nor contrac- 

 tile ; whilst from the centre of the disk there usually proceeds 

 another set of appendages, which sometimes take the form of 

 separate tentacula (as in the accompanying figure), but are fre- 

 quently united into a sort of proboscis which forms a prolonga- 

 tion of the mouth. The body moves through the water by a sort 

 of flapping movement of the disk, which is furnished vs-ith mus- 

 cular fibres. This order is again subdi^dded by Professor E. 

 Forbes (On the British Naked-eyed Medusoe) into two sub-orders, 

 the Steganopthalmata, or hooded-eyed, and the Oymnopthalmata, 

 or naked-eyed ; the former consisting of those which have the 

 oceUi or eye-like bodies of their margin protected by membranous 

 hoods or coverings more or less complicated, whilst the latter 

 have the oceUi unprotected. This character may seem trivial ; 

 but it serves as the indication of a very important difference of 

 internal structure ; for whilst the first of these divisions possesses 

 a much ramified and anastomosing system of vessels spreading 

 over the surface of the disk, the second has a very simple vas- 

 cular apparatus, the circulating canals proceeding to the margin 

 either altogether unbranched, or, if divided, not anastomosing 

 with one another. In the first of these families are included all 



Fig. .-Pel.\gia. the larger Medusoe, such as those belonging to the genera ^ureJw, 



Pelagia, Chrysaora, liMzostoma, Cassiopea, and Cyarnxa ; vvhUst the latter comprehends numerous smaller and 

 more delicate forms, such as those belonging to the genera Oceania, ^quorea, Geryonia, and TlMnmantias. 



II. The form of the body in the Ciliograda is extremely various. Thus in the Cydippe (formerly called Berik) 

 it is nearly globular ; whilst in the Cestiim Veneris it is a long flat riband. The cha- 

 racter of the order, however, is derived from the fact that all the animals composing 

 it are propelled through the water, not by the movement of one part of their bodies 

 upon another, but by the vibration of the ciUa with which certain parts of their 

 surface are covered. In Cydippe the cilia form eight bands, which extend like 

 meridian-lines from pole to pole of the globular body. In Cestum Veiieris, both edges 

 of the long riband-shaped body are fringed with these curious filaments. Notwith- 

 standing the wide difterence in form between the two genera just named, they are 

 connected together by intermediate links. Thus in Callianira, the globular body is 

 extended laterally, so as to form wing-like appendages on either side ; in other genera 

 these appendages are still more extended, and the central globe is lost in them ; 

 until at last the flat riband-like form of Cestum Veneris is attained. The position of 

 the alimentary canal, which has here two orifices, is the same throughout this series ; 

 for whilst in Cydippe it runs from pole to pole of the globe (Fig. 5), in Cestum Veneris •""^''""«' 



it is equally short and straight, running across the body at the middle of its length. In no animal of this order 

 is there anything like an internal skeleton, the whole body being gelatinous. In Cydippe, however, the bands 

 upon which the cilia are seated are of firmer texture than the rest. Many of these animals are very active in 

 their movements, contrasting strongly with the sluggish Pulmograda. The Cydippe pileus, a species very abun- 

 dant on many parts of the British coast, is particularly energetic. It is provided with two long tendril-like 

 filaments, arising from the bottom of two cavities in the posterior part of the body ; and each of them is furnished 

 with lateral branches. These filaments can be entirely retracted within the two cavities of the body, so that 



