ACALEPHjE. 



Rhizostoma ccerulea. 



hollow; and, when connected with the appen- 

 dages of the digestive cavities, or when they have 

 a vesicle at their base, very extensile. Several 

 genera have suckers at the extremities, and 

 along the sides, of their tentacula, by means of 

 which the passing prey is seized. The tenta- 

 cula which are extensile seem to be projected 

 by the forcing of water into their internal cavity, 

 by the contractions of the vesicles at their base. 

 The extent to which the filamentary organ is 

 thus lengthened, in some species, is very extra- 

 ordinary.* It seems to be shortened again by 

 means of the contractions of circular muscles, 

 which force back the water into the vesicle, and 

 of longitudinal muscles which draw it in. 

 Peron thought that some of the pulmograda 

 were furnished with internal air-bladders ; but 

 Eschscholtz, on directing his attention to this 

 point, satisfied himself that what Peron had 

 taken for air-bladders were merely appendages 

 of the gastric cavities, into which air had acci- 

 dentally been introduced during the removal 

 of the animals from their native element. 

 In the cirrigrada, locomotion is effected 

 Fig. 11. 



VelelLa septentrionalis. 



* In the tentacula of some of the physograda, 

 also, a similar extensibility exists. The lower sur- 

 face of physalus, for instance, which itself seldom 

 exceeds six inches in length, is provided with ten- 

 tacula sixteen and even eighteen feet long. 



partly by the movements of the tentacules 

 which hang down from the inferior surface; 

 but chiefly, perhaps, by the action of the wind 

 on the raised crest, with which most of these 

 animals are provided. Immediately around 

 the mouth are placed numerous small tubular 

 suckers, similar to the feet of many echinoder- 

 mata. Exterior to these there are longer tenta- 

 cula, for the most part in a single row, and 

 simple ; sometimes branched. Neither of these 

 two kinds of organs is very extensile. The 

 disc from which the tentacules hang, and the 

 crest, are supported internally by a calcareous 

 plate, which is the only organ of the kind in 

 the whole class of acalephse. It somewhat re- 

 sembles in structure the calcareous axis of 

 retepora, being cellular and porous. Its nu- 

 merous cells are filled with air, which renders 

 the whole animal so buoyant that it floats on the 

 surface of the water, and is wafted along by the 

 winds. In velella (Fig. 11.) there are two 

 plates, one placed horizontally, the other perpen- 

 dicularly upon the upper surface of the former. 

 They are marked with lines of growth, enlarg- 

 ing from within outwards, like the extravascular 

 shells of the inollusca. The perpendicular 

 plate in velella supports the crest, which stands 

 upright, and exposes a large surface to the wind. 

 Rataria (Fig. 12.) has its crest provided with 

 strong muscular bands run- p. 



ning perpendicularly. It 

 lies on the surface of the 

 water, with the crest stretch- 

 ed out, so that its whole side 

 touches the water. When 

 it is alarmed, the crest is 

 suddenly contracted, and the 

 centre of gravity is so al- 

 tered in consequence, that 

 the position of the body is almost reversed. 

 When the crest is again raised, the body imme- 

 diately resumes its former position. 



Porpita has a simple plate supporting its disc, 

 without any crest, and long tentacula, which are 

 so delicate as scarcely to bear the slightest touch 

 when the animal is taken out of the water. When 

 the position of the animal is altered by the hand, 

 so as to make the surface covered with suckers 

 the upper one, all the tentacula of one half of 

 the body turn round to the dorsal surface, and 

 all those of the other half stretch over their 

 own surface, and thus the animal very soon 

 regains its old position. 



II. Motility and Sensation. Almost all 

 observers have failed to discover anything re- 

 sembling a nervous system in the Acalephae. 

 Even Eschscholtz,* who devoted so much atten- 

 tion to their anatomy, could not see nerves in 

 the largest that he examined. But in Cydippe, 

 according to Dr. Grant,f there is a structure 

 which can be regarded only as belonging to 

 the nervous system. It consists of a double 

 transverse filament of a milky white colour, 

 running round the body, near its surface, at a 

 short distance above the mouth. The two 

 cords of which this filament is composed unite 

 in the middle of each of the spaces between 



* System, p. 19. 



t Trans. Zool. Soc. of Loud. i. 10. 



Rataria cordata. 



