RADIATA. 



C95 



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 

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



I. The order Felmograda, or Discophor/r, including all the ordinary Meduscv, 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 proceed! 

 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 witli mus- 

 cular fibres. This order is again subdivided by Professor E. 

 Forbes (On the liritish N'aked-eyed Medusa?) into two sub-orders, 

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

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

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

 hoods or coverings more or less complicated, whilst the latter 

 have the ocelli unprotected. This character may serin 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 

 the larger Medusa;, such as those belonging to the genera .1 



Tclagia, Chrysaora, Rhizostoma, Cassiopea, and Cyarura ; whilst the latter comprehends numerous smaller and 

 more delicate forma, such as those belonging to the genera Oceania, /Equorea, Qtryonia, ami Than 



II. The form of the body in the Ciliogkada is extremely various. Thus in the Cydippt (formerly called Bcroe) 

 it is nearly globular ; whilst in the ' turn I. nerit it is a long flat riband. The cha- 

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

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

 upon another, but by the vibration of the cilia witli which certain parts of their 

 BUrface are covered. In Cydippt the cilia form eight bauds, which extend like 

 meridian-lines from pole to pole of the globular body. In Csstum Vtntril, both edges 

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

 standing the wide diilercnce iii form between the two genera just named, tiny are 

 connected together by intermediate links. Thus in CMiani™, the globular bod] is 



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

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

 until at last the flat riband-like foi tin \,ifn.< is attained. The position of 



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



for whilst in ( "ydippi it runs from pole to pole of the globe (Fig. 5), in C'estum V< 



it is equally Short and straight, running <t.-ro.».< the body at the middle of Its length. In DO animal of this ordi r 



is there anything like an internal skeleton, the whole body being gelatinous. In Cydippt, how. mi-, the hands 

 upon which the cilia are seated are of tinner texture than the rest. Man] of th.seannn.iN an verj i 

 their move m e n ts , contrasting strongly with the sluggish Pulmograda. The <>/ ->.r_\ abun 



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

 til mi. nt-, arising from the bottom of two cavities in the post i lor pa rt of the I od] i and i acta of them is ruruished 



With lateral branches. These filaments can be entire!] retracted within the two c.r.ities of the body, SO that 



I i,. S. ( i 1'irrn : o, c, • ■ 

 ith | '■, lam 



