EADIATA.— ACALEPH^. 



6S3 



CLASS ACALEPH.E. 



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 ba-sed 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 Physoorada. 



I. The order Phlmoghada, or Discopnon^, including all the ordinary Medus/p, Is characterized by the re^lar 

 discoidal or circular form of the animals composing it. The body is of gelatinous texture, Tvithout 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 -nith 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, wliieh is fm-nished with mus. 

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

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

 the Steganopthalmnia, 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 covei-ings more or less complicated, whilst the latter 

 have the ocelli unprotected. This character may seem tririal ; 

 but it serves as the indication of a very important diflference 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 Medusoe, such as those belonging to the genera Aureliu, 

 relagia, Chrysaora, Bhizostoma, Cassiopen, and Cyaruva ; whilst the latter comprehends numerous smaller and 

 more delicate forms, such as those belonging to the genera Oceania, ^quorea, Geryonui, and Thaumantia!, 



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

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

 racter of tlie 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 cilia 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 Veneris, botli edges 

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

 standing the wide difference 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 Venxeris 

 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 

 npon which the cilia are seated are of firmer texture than the rest, ilany of these animals are very active iu 

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

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

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

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



Fig. 4.— I'Ei.AGiA. 



Fig 6. Cvdippe: a, n, unu 

 culn-, b, luoutti J c, [criuinai of 



intest.ne. 



