696 



RADIATA. 



tliey are not visible externally ; and arc put lorth at tne -n-ill oi tne animal, the main filaments being first ejected, 

 apparently by the eontractiou of the cavity, and the lateral tendrils then uncoiling. 



III. The CtRiiHiGEADA form a small group, distinguished by the presence of a cartilagino-as internal skeleton, 

 and by the possession of very numerous contractile cirrhi or tcntacula surrounding the mouth by the movements 

 of which the animal is slowly propelled through the water. In Porpita, the body forms a circular disk, and the 

 cartilaginous skeleton is flat. In Velella the body is oval, and the cartilaginous disk has a Tertical plate rining 

 from it, which acts as a sail when these beautiful little animals are floating on the surface of the water. The 

 stomach in these animals is a simple flask-like cavity, placed under the centre of the disk, and having but one 

 orifice, which is furnished with a sort of proboscis. 



IV. The order Phtsograda correspunds with the Hydrostatica of Cuvier. In this group we lose altogether the 

 radiated iOTVCi, but have a lateral symmetry; the two halves of the body, divided by a plane passing from one 

 end to the other, being similar to each other. The anatomy of the Fhysalia has been carefully investigated 

 of late, and has been proved to be in some respects different from the description given by Cuvier. There is a 



L pruper digestive cavity, entirely distinct from the air-sac, and situated underneath 



one of its extremities ; this has no single mouth, but receives its supplies of aliment 

 through a number of flask-shaped appendages which hang down beneath, each 

 having an orifice at its extremity, surrounded by a sort of sucker. These are en- 

 tirely distinct from the long contractile tentacula, which are employed for grasping 

 prey, at the same time paralyzing it by means of their peculiar stinging power 

 These tentacula can be drawn up to within half-an-inch of the air-bladder, and ma .■ 

 be then suddenly shot fortli to a length of eighteen or twenty feet. In this manner 

 they attack small fishes, even at a considerable distance, and then probably draw 

 them within reach of the suctorial appendages. It is not peculiar to this animal to 

 have the single mouth replaced by numerous minute orifices ; for the same is the 

 case with the Rhizostoina among the Pulmograda, 



A most unexpected connection has been recently discovered between the Pulmo- 

 grade AcaUphi:, and the Hydraform Polypes; which shows that the two classes, how- 



Fic. r>.--ppv3ALiA I a r, orifices evcr dissimilar in their aspect and structure, are very closely united to each other, 

 crest. ^ ^^^ ^^ ' ' "^'" '^°''*'"^ and should even be included in the same group. It has been ascertained that many 

 species of the Pvlmograd^ Jcalepho!, both hoodcd-e-yed and wifceii-ei/etf—aud therefore, probably, the whole order- 

 begin life in a irM(^ polypoid state, and only acquire the Mcdasan character after a series of very remarkable meta- 

 morphoses. On the other hand it would appear that many of the animals known as IJydraform Polypes produce, 

 by gemmation, bodies which are in all respects true Medusae, and which are charged with the production of ova, 

 from which a new generation of Polypes shall arise. The latter part of this interesting series of phenomena 

 w-ill be described under the head of Polypifera ; of the development of the Me<ius'j?, as made knowu by the ubser- 

 vations of Sars, Siebold, Steenstrup, and Sir J. G. Dalyell. a brief account will now be given. 



From the egg of the Afedusrc is first produced a minute disk, very much resembling an infusory animalcule, 

 and moving through the water by the action of the cilia with which its body is fringed (Fig. 7, a) ; at one ex- 

 tremity of the body is a minute depression, by which it afterwards becomes attached. After about three days, 

 the embryo attaches itself to some fixed object 

 (as at h) ; the form of the body begins to change 

 from the cylindrical to the club-shaped (c. d) ; 

 and the cilia of its surface disappear. The upper 

 end is now flattened ; the position of the mouth 

 is marked out by a depression in its centre, 

 which is surrounded by an elevated margin ; and 

 four indistinct tubercles, the rudiments of ten- 

 tacula, are seen around it. (This is shown at e, 

 \^hich represents an individual in the condition 

 of d, but seen from above.) The tubercles gra- 

 dually elongate into tentacula ; a true month is 

 seen in the centre, and tentacula spring up be- 

 tween the preceding ; and the body gradually 

 assumes the form of the Hydra. These changes 

 are represented in Figs. /, g, h, i, j, and k ; Fig. 

 g, being a view taken from above of the animal 



the stage /; and Fig. k being a corresponding 

 view of the stage j. Now in this condition, the , 



Fio. 7. — Developme: 



T OF niKDl"iA ( (7. h, e, ^-r, 

 polypi. id growth. 



nimal is in every essential particular, a true Polype ■ and has 

 been repeatedly so described. It remains attached by its base to one spot, draws its food into its mouth by 

 means of its arms, and these contract when the stomach is distended, and cannot then be irritated to movement 

 Not only does it live as a Polype, but it also reproduces itself as a polype ; for polype-buds are not unfrequentiv 

 seen to issue from its sides {Fig. 7, I) ■ these become detached and form new individuals, just as in the Hydra Thus 

 from a smgle individual, a whole colony may be produced ; and these may all continue in the polypoid condition 

 for many months, or even years. But under some peculiar circumstances, whose nature has not yet been deter 

 mined, an entirely new aeries of changes at last takes place. The body assumes a more elongated cylindrical form' 



