684 



Div. 4. RADIATA.— ACALEPII^. 



they are not visible externally ; and are put forth at the will of the animal, the main filaments being first ejected, 

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



III. The CiBKHioRADA t'orm a small group, distinguished by the presence of a cartilaginous internal skeleton, 

 and by the possession of very numerous contractile cinhi or tentacula 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 vertical plate riuing 

 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 Phtsoqeada corresponds with the Hydrostatica of Cuvier. In this group welose altogether tbe 

 radiated form, 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 Physalia has been carefully investigated 

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



1 proper 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 balf-an-inch of the air-bladder, and may 

 be then suddenly shot forth 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 Rhizostoma among the Pulmograda. 



the 



crest. 



A most unexpected connection has been recently discovered between the Piilma. 

 grade Acalepha;, and the Bydraform Polypes; which shows that the two classes, how. 

 Fio. C— Physalia < a c, orifices ever dissimilar in their aspect and structure, are very closely united to each other, 

 air-sac, b, '°«'"''™"™'« g^„j s,n,u,,l gyg^ bg i„clu(jed in the same group. It has been ascertained that many 

 species of the Pulmograde Acalcph<c, both hooded-eyed and naked^yed-and therefore, probably, the whole order- 

 begin life in a trne polypoid state, and only acquire the Mcdusan character after a series of very remarkable mcta- 

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

 by gemmation, bodies which are in all respects true Medusse, 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 

 will be described under the head of Polypifcra ; of the development of the Medusa;, as made known by the obser- 

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



From the egg of the ^fedusai 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, 

 which represents an individual in the condition 

 of rf, but seen from above.) The tubercles gra. 

 dually elongate into tentacula ; a true mouth is 

 seen in the centre, and tentacula spring up be- 

 tween the preceding ; and the body gradually 

 assumes the form of the Uydra. 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 st.ige j. Now in this condition, the animal 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 he irritated to movement. 

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

 seen to issue from it.s sides (Fig. 7, I) ; these become detached and form new individuals, just as in the Hydra. Thus 

 from a single individual, a whole colony may be produced ; and these m.iy 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 .series of changes at last takes place The body assumes a more elongated cylindrical form 



Fio. 7.— DEvr.i,oPMI!NTorMp.DIT«A; n, h,e,^c.) successive stages of 

 jiolypdid ^owth. 



