44 



ACALEPHjE. 



the other. They then terminate in another 

 annular vessel, which surrounds the mouth. 

 In their course they give off numerous 

 branches. From the oral circle of vascular 

 structure arise two large vessels, which run 

 along the walls of the gastric cavity, and ap- 

 pear to unite with the other circle at the anal 

 extremity. These last Eschscholtz regarded as 

 veins, and the eight external vessels as arteries. 

 He supposed that the veins, passing along 

 the walls of the stomach, absorbed the nutri- 

 ment, and then carried the circulating fluid to 

 the cilia for aeration. In the course of his 

 observations on the Beroe ovatus, Dr. Fleming* 

 distinctly saw a fluid moving " backwards and 

 forwards" in the external vessels ; and he states 

 that " while the animal was active, there were 

 numerous small spaces in the different vessels 

 where the contained fluid circulated in eddies." 

 Dr. Fleming failed to detect any structure in 

 the vessels which could produce these partial 

 motions. In cestum naiadis, Eschscholt/ thought 

 that he saw the system of vessels more dis- 

 tinctly than in any other of the acalephae. He 

 thus described it : " From the base of each of 

 the two tentacules, a vessel takes its rise, and 

 goes towards the bottom of the stomach. Here 

 the two vessels unite, and form a little vascular 

 circle around the water-canal (intestine). From 

 the upper margin of this circle, four straight 

 vessels arise, which go towards the two rows of 

 cilia-bearing organs placed on the dorsal sur- 

 face. Under these they run, two in one di- 

 rection, and two in the other. At either 

 extremity of the body, these unite with certain 

 vessels running superficially along the sides, 

 and which complete the circulation by entering 

 the first set of vessels just before they 'begin to 

 run beneath the ciliated organs. All these 

 vessels are simple canals, of the same diameter 

 throughout, without any visible branches. They 

 contain a colourless watery fluid, in which mi- 

 nute yellowish globules are seen to move. In 

 the vessels which arise from the bases of the 

 tentacules, the globules mount upwards ; they 

 assume a rotatory motion in the vascular circle; 

 and, in the four dorsal vessels, they seem to 

 move, some in one direction, others in the other. 

 It is probable that what appears to the eye as 

 one vessel, is, in reality, composed of two 

 vessels, running parallel and close together."f 



Seeing that the radiating vessels which arise 

 from the gastric cavities of the pulmograda 

 seem to carry out the nourishing material to all 

 parts of the body, and that they are, in some 

 species at least, connected with other vessels 

 which form a complete circle, we are disposed 

 to class them under this head along with the 

 vascular structures already described. The 

 exact analogies of their functions, however, 

 have not yet, we conceive, been distinctly 

 made out. 



From the stomach of rhizostoma, formerly 

 described, sixteen vessels arise, and pursue a 

 straight course outwards to the margin of the 

 disc, near which they all enter, at equal dis- 



* Mem. Wcrn. Soc. iii. 401. 

 t Op. cit. p. 14. 



tances, a circular vessel, which passes com- 

 pletely round the circumference of the animal. 

 Four of the radiating vessels correspond with 

 the four fleshy pillars of the process supporting 

 the arms, and there exists on the internal sur- 

 face of each of these pillars, a groove, which 

 establishes a direct communication between the 

 corresponding vessel, and one of the large 

 vessels of the central process. The other 

 twelve are distributed by threes in the intervals 

 between the first four, and arise from those 

 parts of the stomach which are closed by the 

 plaited membranes. The space intervening 

 between the circular vessel and the margin of 

 the disc is occupied by an innumerable multi- 

 tude of little vessels which form a net-work 

 like the finest lace.* In medusa aurita, there 

 are also sixteen radiating vessels, four of which 

 arise from each of the four sacs, into which the 

 gastric cavity in this species is divided. Two 

 of the four vessels in each group are simple, 

 the other two are several times bifurcated ; 

 both the simple main trunks and all the 

 branches so formed, enter a circular vessel sur- 

 rounding the disc, which seems to be connected 

 also with the tubular cavities of the numerous 

 cilia which surround the margin like a fringe, 

 and which are capable of elongation and con- 

 traction.-f- Carus remarks with regard to the 

 circular vessel, that " it may be considered as 

 an extremely simple rudiment of the great cir- 

 culation of superior animals, in case we view 

 the radiating as chyliferous vessels.''^ 



V. Respiration. It is probable that the air- 

 bladders of the physograda, the swimming 

 organs of the diphyda, and the cilia of the 

 ciliograda are all subservient, in a greater or 

 less degree, to the respiratory function, as well 

 as to locomotion. The vessels in the last men- 

 tioned class, which have been described above 

 as appertaining to the circulating system, are 

 regarded by some as respiratory organs ; and 

 by Lamarck were compared to the tracheae of 

 insects. They have been called aquiferous 

 trachea. Those who consider them in this 

 light believe that they are open at two points, 

 so as to admit the circumambient fluid to pass 

 freely through them. The most recent and 

 accurate observations, however, leave it doubt- 

 ful whether this really takes place in the 

 ciliograde acalephae. 



With regard to the pulmograda, several 

 parts and organs have been pointed out by 

 different observers as being, in all probability, 

 the seats of the respiratory function. Cuvier 

 thought that the delicate plaited membranes 

 which exist between the fleshy pillars of the 

 central process in rhizostoma, and which form 

 in part the walls of the stomach, might be re- 

 garded as the organs of respirarion. Eisenhardt 

 supposed that he saw them in certain tentacu- 

 lated processes attached to the membranous 

 partitions which divide the gastric sacs of some 

 species from one another ; while Gaede looked 

 upon the four small sacs which overlie the 



* Cuvier, Journ. de Phys. xlix. 433. 



t Gaede, Anat. der Medusen. 



$ Carus, Comp. Anat. (by Gore,) ii. 266. 



