CIRCULATION. 



653 



Fig. 326. 



Lobster. 



within these sacs, as upon the gills or lungs of 

 other animals, but the exact course of the blood 

 does not appear as yet to have been satisfacto- 

 rily ascertained in these animals. Audouin* 

 believes it to be essentially the same as in the 

 Crustacea. The long-shaped dorsal vessel or 

 heart gives off arteries to both sides, and re- 

 ceives at one place branches from the gills. 

 The veins form only spaces or sinuses, and not 

 vessels on the abdominal side of the animal. 

 The blood propelled from the artery is passed 

 through the system, returning from which, it is 

 collected into the venous sinuses below, thence 

 it proceeds to the pulmonary organs, and after 

 passing through them, returns to the heart. 



Zoophytes. The general character of the 

 circulation in this class is exceedingly ob- 

 scure ; for while in some of the animals be- 

 longing to it, comparative anatomists have not 

 succeeded as yet in pointing out any distinct 

 vascular system ; in others, they have been at a 

 loss to determine, among various vascular or- 

 gans, which of them forms the proper circula- 

 tory system corresponding with that of higher 

 animals. 



Echinodermata. Among the Zoophytes the 



Echinodermata present the most fully deve- 

 loped vascular system with which we are ac- 

 quainted. According to the observations of 

 Tiedemann and Delle Chiaje, who have inves- 

 tigated the structure of these animals with great 

 success, there are two principal divisions of the 

 vascular system, described by the first of the 

 above-mentioned authors as distinct from one 

 another, by the other as communicating toge- 

 ther. 



We do not feel inclined to consider, in ac- 

 cordance with the view of these authors, that 

 series of cavities which is employed in loco- 

 motion as a part of the nutritive circulatory 

 organs. 



That part of the vascular system of these 

 animals again, which is situated in the neigh- 

 bourhood of the alimentary canal, very proba- 

 bly corresponds with the circulatory organs 

 which we have been describing in other ani- 

 mals; since arteries and veins can be distin- 

 guished in it, and there is good reason to be- 

 lieve that a circulation of fluid takes place 

 through its vessels in all the kinds of Echino- 

 dermatous animals. 



In the Holothuria, the principal artery or 

 heart is connected with a ring situated round 

 the commencement of the alimentary canal, 

 from which the systemic arteries are given off: 

 the systemic veins send branches to the gills, 

 and the returning vessels from these organs 

 transmit the circulating fluid through one large 

 trunk into the heart. 



The intestinal vascular system of the Asterias 

 and Echinus is somewhat similar to that of the 

 Holothuria, consisting of annular vessels, from 

 which arteries and veins are given off, and con- 

 nected with a dilated contractile canal, consi- 

 dered as a heart. 



Planaria. Next to the Echinodermata in 

 respect of the degree of perfection of their cir- 

 culatory organs, may be mentioned the Plana- 

 riae, in which M. Uuges* has pointed out a 

 very remarkable system of vessels which ap- 

 pear to constitute circulatory organs (fg. 327, 

 a, a ). For some time previously to the disco- 

 very of these vessels, the sin- 

 Fig. 327. gularly branched intestinal ca- 

 vity of the Planaria and some 

 Entozoa was believed to hold 

 the place of organs of circula- 

 tion, the same cavity in which 

 digestion occurs being believed 

 to carry by its ramifications the 

 nutritious fluids to different 

 parts of the body. But Duges 

 has shewn the existence in them 

 of a system of vascular organs 

 resembling considerably those 

 of the Leech, to which animals 

 the Planaria bears, in other parts 

 of its organization also, astriking 

 analogy. The vascular system 

 Planaria. of the Planaria consists of three 

 principal longitudinal trunks, 

 two lateral and one dorsal or median, which are 

 all united together by numerous minute anasto- 



Sce the article Arachnida, p. 206. 



* Annal. dcs Sciences Natur. xv, p. 160. 



