98 



SRA-XKITFiKS. 



and the Diphyida, which consist of two parts most delicately joined, and 

 whose motion is produced by the contraction and expansion of the halves 

 which are constantly ejecting and imbibing circumambient fiuid. The 

 Puhnonigrada possess in common a gelatinous disc, which, however, upon a 

 section being made, shews the variety of texture, nor is there any fibre appa- 

 rent by which any motion in itself could be given to it. This disc has the 

 shape of a mushroom, or a belt, which floats with its concaA^e part down- 

 wards, and contains within it the body of the Pidmonigrade, which is very 

 similar in its structure to the root of a plant, and from which numerous 

 tentalula emanate, not unlike the leaves of the common Kale, with small 

 plain leaflets pendant at the extremities. 



In the Bhizostoma there is a double row of these leaves by which the food 

 is collected and absorbed. The Ciliograda are of different forms; some being 

 circular like a sphere, and others mere ribbon-like substances; but in all the 

 ciliated bands are the means of locomotion; and it would appear that the 

 animal has the most perfect control over their movements, as much as we 

 have over a finger, or perhaps more; for the discontinuance, or sudden action 

 of any of these cilia, as occasion may require, either promote or retard the 

 action in the water, which may be regarded as more allied to vibration than 

 actual motion, being very quick, and to our vision scarcely recognisable. 



In some species these bands lie on the surface; in others they are between 

 folds of membrane, which, at times, perfectly and entirely conceal them, and 

 this is probably for their protection whilst the creature is inactive. 



The agent employed in the motion of these numerous paddles is still a 

 mystery, although it has been supposed that a fluid is secreted beneath them, 

 which inflates their tubular substance, and causes them to project; but this is 

 all that is at present known, although some species are of considerable size. 

 The most familiar example of the flat-shaped creature, which I have before 

 referred to, is the Cestum Veneris. In this species the cilia are placed upon 

 the edge only like a fringe, in a double row, of the most lovely hues imag- 

 inable in the sun-light, but which, when dai-kness veils the surface of the sea, 

 appear highly phosphoric, and emit a most brilliant flashy light. Some of 

 these animals are many feet in length, and under the cilia, for the whole 

 length of the medusa a canal is observed to pass, which is supposed to 

 secrete a fluid for the purpose of erecting the cilia, as in the case of the 

 other Cdiograda. 



The next division of the Acale.phce is the Physo^rada, an animal composed 

 of a bladder-like substance, if that word can be so applied. The familiar 

 instance of these animals, as I have before observed, is that beautiful creature, 

 the Portuguese Man-of-War, which is so often mentioned by voyagers to the 

 East. The shape of the bladder-like body of the medusa is elliptical, with 

 a small additional pear-shaped smaller protuberance at one end, bearing to 

 the body somewhat the relation of a head, at what may be termed the 

 neck of this additional bladder, or where it is contracted; as well as at the 



