650 



CIRCULATION. 



are acquainted. Mr. Lister finds that the dif- 

 ferent Ascidise of a branched animal are not only 

 connected together by the polypiferous stem, 

 but have a common circulation. In each indi- 

 vidual there is a heart consisting of one cavity 

 only, and pulsating about thirty or forty times 

 in a minute. In the common stem, the mo- 

 tion of the globules of the blood indicates 

 distinctly two currents running in opposite 

 directions. One of the currents enters the 

 Ascidia by its peduncle and proceeds directly 

 to the heart ; the blood issuing from the heart 

 is propelled into the gills as well as the system 

 at once, and upon its return from thence the 

 returning current proceeds out of the animal 

 by its peduncle again into the common stem, 

 whence it goes to circulate through another of 

 the ascidiae attached to the stem. The direc- 

 tions of the currents appeared to be reversed 

 every two minutes or less. According to Mr. 

 Lister, when one of the ascidiee is separated 

 from the common stem, its circulation goes on 

 in an independent manner; the blood return- 

 ing from the body being conducted into the 

 heart, but the alternation of the directions still 

 continues, a circumstance which points out an 

 important difference between the compound 

 and the simple ascidiae, in which last the cir- 

 culating fluid is generally believed to pass from 

 the gills into' the heart, and to hold continually 

 the same direction. 



Articulata. In this class of animals, 

 varied as the forms of the circulatory organs 

 appear, the position of their principal parts is 

 much more constant than in the Molluscous 

 animals. In some, as the Decapodous Crus- 

 tacea, there is a short and thick muscular heart 

 connected with the systemic arteries. In 

 others, the contractile part of the vascular 

 system is much more like a dilated artery than 

 a circumscribed heart, as occurs in some other 

 Crustacea, spiders, and insects; and in the 

 Annelida the greater part of the large vessels 

 seem to be endowed with a contractile power 

 by which they propel the blood. 



Annelida. Although the Annelida form the 

 highest division of the class Articulata in the 

 arrangement of Cuvier, their circulatory organs 

 may for the most part be regarded as more 

 simple than those of most of the others. The 

 circulation is best known in the J^aides, the 

 Leech, Earthworm, and Sand worm. In all of 

 these, the blood, which is generally red, moves 

 gradually forwards in the vessels situated on 

 the upper surface of the animal, and backwards 

 in the vessels placed below or on the abdomi- 

 nal side. There are also numerous cross vessels 

 which transmit the blood from one side to 

 another, or from above downwards, or from be- 

 low upwards, in each of the compartments or 

 joints of the animal. The upper vessels, being 

 generally the most contractile, are considered 

 as the arteries ; the lower vessels as veins.* 



The organs of circulation appear to be sim- 

 plest in the Naides. In these animals, the 

 contractile part or heart is represented by an 

 artery above. This vessel turns round at the 



* See the article Annelida, p. 169. 



head into the vein which is below. The artery 

 sends its blood partly into the gills, placed 

 along the whole length of the body, from which 

 it again receives the returning blood, and by 

 numerous lateral branches, which may be re- 

 garded as the only capillary vessels, it sends 

 blood across the body of the animal into the 

 vein. The motion of the blood appears to be 

 partly progressive and partly oscillatory. 



Lumbricus. In the common earthworm, 



there are two principal vessels, the one (Jig. 



323, a,) placed above and the other (v) below, 



and extending the 



Fig. 323. whole length of 



the body ; these 

 two principal ves- 

 sels communicate 

 together by very 

 numerous small 

 cross branches (c), 

 and, in the neigh- 

 bourhood of the 

 ovaries, by from 

 five to eight very 

 remarkable neck- 

 lace-shaped or 

 moniliform ves- 

 sels (h, H). At 

 the place of junc- 

 tion of these mo- 

 niliform vessels 

 with the lower 

 longitudinal one, 

 there are small di- 

 latations of that 

 vessel, which are 

 believed to aid in 

 propelling the 

 blood by their con- 

 tractions. There 

 are also three other 

 longitudinal ves- 

 sels, much smaller 

 than the principal 

 or median ones, 

 which join with 

 Lumbricus. the cross anas- 



tomosing twigs. 



The upper principal vessel pulsates in an un- 

 dulatory manner, the contraction taking place 

 first at the posterior part, and proceeding gra- 

 dually forwards. In these animals, however, 

 the course of the blood does not appear to be 

 very well known. It is believed to be from 

 behind forwards in the upper vessel and from 

 before backwards in the lower, but there must 

 be also lateral motion. Both the upper and 

 lower vessels are said to give off pulmonary 

 branches. 



Arenicola. In the sandworms also, besides 

 the principal upper and lower vessels, there are 

 two smaller ones, placed one on each side of the 

 abdominal nervous cord, and two others upon the 

 intestine; between these there is a very minute 

 net-work of smaller branches. The branchial 

 arteries are derived from the upper longitudinal 

 vessel, the branchial veins lead into the lower. 

 The greater part of the blood proceeds from the 

 upper vessel into the gills by the branchial arte- 



