RESPIRATION AND CIRCULATION. 33 



here in error, as indeed he himself subsequently admits. Meyen asserts the exist- 

 ence of an aperture in the vicinity of the anus, through wliich, he tells us, he has witnessed 

 the escape of an egg in Alcyonella ;* and Siebold admits the correctness of this statement, and 

 considers the aperture described by Meyen to be that through which the external water is 

 admitted to the interior.f I have, however, fully convinced myself that no such aperture 

 exists, and the phenomenon described by Meyen must certainly be due to an accidental rupture 

 of the tissues, though Van Beneden describes the passage of the eggs through an aperture 

 similarly placed in the marine genus Laguncula.\ It is possible that certain apertures may 

 exist in some of the tissues of the animal so minute as to defy our attempts at detection, and 

 yet capable of permitting a passage of fluid ; some facts already recorded (page 13), would 

 seem to point to the existence of a system of tubes in the substance of the endocyst, which 

 may afford the necessary channels of communication, or it may be that it is in simple 

 transudation through the walls of the alimentary canal that we are to seek for the true mode 

 in which the external water passes into the perigastric space. 



The real signification of the perigastric fluid is a point whose determination must be of 

 great importance in the physiology of the Polyzoa. As lias just been said, it is by no means 

 homogeneous, and numerous corpuscles of very various and irregular shape may be observed 

 to float through it and be carried about by its current. Some of these corpuscles are, doubt- 

 less, spermatozoa ; others are of no definite shape, and look like minute portions of the tissues 

 separated by laceration. 



If it be admitted, as I think it must be, that the perigastric fluid consists mainly of water 

 which has obtained entrance from without, it then corresponds to a true aquiferous system 

 subservient to a respiratory function. But it also without doubt receives certain products of 

 digestion which had transuded through the walls of the alimentary canal ; it thus connects 

 itself with the digestive system. It is, moreover, , the only representative in these animals of 

 a sanguiferous circulation, for in the Polyzoa there is certainly no trace of a heart, nor can 

 anything referable to a true vascular system be detected. The perigastric circulation, there- 

 fore, unites in itself the triple function of a chyliferous, sanguiferous and respiratory system. 

 And the fluid in question would correspond to the " chyle-aqueous fluid," which plays so 

 important a part in the economy of the lower animals, and whose nature has been recently 

 well elucidated by the researches of Dr. Williams. § 



The next point of interest to determine, with regard to the perigastric fluid, is the cause 

 of the peculiar currents observed in it. These currents, which extend into the tentacular 

 crown, were long ago observed by Trembleyll in LopJiopus crptaUinus ; but this author con- 

 tented himself with simply recording their existence, and made no attempt to explain them. 

 Nordmann,^ who observed them both in fresh water and marine genera, not being able to 

 detect any trace of cilia or other moving power, compared them to the currents in the cells of 

 Chara. That they are produced partly by the action of vibratile cilia, and partly by the muscular 



* Meyen, Naturgeschichte der Polypen, Isis, 1828. t J-'OC. cit. § 41. 



I Recherches sur I'Org. des Laguncula, ' Nouv. Mem. de I'Ac. Brux.,' 

 ^ Oil the Bluod proper and Cliyle-aqueous Fluid. ' Phil. Trans.' 1852. 



II Mem. pour I'Hist. des Polypes d'eau douce. 

 ^ Microgiaphische Beitrage, Bd. ii. p. 75. 



