654 



CIRCULATION. 



mosing vessels. The larger parts of the longi- 

 tudinal vessels have been observed to contract 

 and dilate; but neither a regular progressive 

 circulation, nor a connection of the vascular 

 with any distinct respiratory system has as yet 

 been detected. 



Entozoa. In the Entozoa, organs of circu- 

 lation somewhat similar to those just mentioned 

 in the Planarise have been found by Bojanus 

 and Mehlis in the Distoma and Tristoma, 

 and by Nordmann* in those remarkable small 

 Entozoa inhabiting the aqueous chamber of 

 the eyes of some quadrupeds, the Diplosto- 

 mum, and in the Diplozoon. In the first of 

 these animals, the motion of fluid in the vas- 

 cular system is exceedingly obscure; but in 

 the Diplozoon (Jig. 328), Nordmann saw, with 



Fig. 328. 



DiploKOon. 



a high magnifying power, currents moving in 

 opposite directions in two sets of vessels (a, v) 

 placed on each side of both limbs of the ani- 

 mal. These vessels, termed external and in- 

 ternal, are said to terminate posteriorly in a 

 dilated bag, to which Nordmann gives the 

 name of receptacle of the chyle. The organs 

 of circulation of the Diplozoon differ, there- 

 fore, in this respect from those of the Plaparia, 

 to which otherwise they bear considerable si- 

 milarity; for, in the latter animal, the vascular 

 system appears to be entirely closed. Accord- 

 ing to Nordmann and Ehrenberg no contrac- 

 tions or dilatations of the vessels are visible. 



Acalepha. In some of the Medusa tribe, 

 or Acalephae, there appears to be no distinct 

 circulatory apparatus; and we observe that in 

 these instances, the alimentary cavity is of 

 great extent and is often much ramified on the 

 surface of the animal. 



In others there are distinct vessels with a 



* Micographische Beitrage, p. 69. Berlin, 1832. 



circulation of fluid within them. The distri- 

 bution of this very simple kind of vascular 

 system was first discovered by Eschscholtz, who 

 has described its form particularly in the Cesium 

 and Beroe. In the latter animal, it is stated 

 that eight arterial vessels and two veins unite 

 with a large annular vessel which surrounds the 

 mouth, and, according to Eschscholtz's* conjec- 

 ture, another vascular ring, situated at the pos- 

 terior extremity of the body, forms the means 

 of communication between the arteries and 

 veins in that region. Branches pass from the 

 external or arterial vessels, and from the in- 

 ternal or venous vessels to the fins, which 

 organs seem to serve at once for respiration 

 and for locomotion. Although the motion of a 

 yellowish fluid containing globules has been 

 seen in these vessels, the complete circulation 

 does not appear to have been made out in 

 a satisfactory manner. 



Infusoria. Some kind of circulation is 

 stated to have been observed by Ehrenberg in 

 some of the Infusoria ; but this is an observa- 

 tion which, with every confidence in the ac- 

 curacy of this celebrated microscopic observer, 

 we feel inclined to consider as liable to fallacy, 

 on account of the prevalence of various kinds 

 of ciliary currents in the interior of many of 

 these animals. 



Polypi. We would extend the same remark 

 to the last kind of circulation to which we 

 shall allude, viz. those singular currents of 

 fluid, which were discovered by Cavolini and 

 recently observed by Mr. Lister in some of 

 the Polypiferous Zoophytes. According to 

 the latter observer, in each of the divisions 

 of the stem of the Tubularia indivisa, a cur- 

 rent of fluid carrying globules along with it 

 is seen proceeding up one side and down the 

 other. In various Sertulariae, the direction of 

 the current becomes reversed from time to 

 time. Similar phenomena are to be observed 

 in Campanularise and Plumularise. The 

 striking analogy which these currents bear to 

 those occurring in the stems of some plants, as 

 Chara and Caulinia, seem to us to bring them 

 under another class of phenomena than those 

 of the vascular circulation of the higher ani- 

 mals. We do not, however, intend to enter 

 upon the consideration of this subject, as it 

 is already fully treated of under the article 

 CILIA. 



In concluding our notice of the simpler 

 forms of the circulatory organs, we would re- 

 mark that one of the great difficulties which 

 retards the acquisition of an accurate know^ 

 ledge of the function of circulation in the 

 lowest classes of animals, proceeds from our 

 inability to determine, whether currents moving 

 within enclosed spaces in these animals belong 

 to the circulation of their blood and nutritious 

 fluids, or are connected with respiration, loco- 

 motion, and other processes of their economy ; 

 and this is an obstacle to the progress of the 

 investigation which from its nature we cannot 

 hope soon to see removed. 



* System der Acalephen. Berlin, 1829. See 

 the article Acalepho-, p. 43- 



