SANGUIFEROUS SYSTEM IN THE INVERTEBRATA. 85 



Diplo-neura. — In the earth-worm two vessels, one above and the 

 other below, extend the entire length of the body; they communi- 

 cate by several cross branches, and join at their extremities, where 

 small dilatations are observable, supposed to aid in propelling the 

 blood, which moves in opposite directions. The large vessels of the 

 annelida are endowed with a contractile power, and in insects, 

 spiders, and the lower species of Crustacea, large dilatations occur, 

 capable of considerable contractility. 



In the leech, hirudo vulgaris, the two principal vessels are placed 

 on the sides, and lesser ones above and below, all anastomosing 

 freely by transverse branches ; of these the superior longitudinal 

 and the inferior transverse only pulsate. It is the opinion of 

 Miiller that the lateral vessels alternately empty themselves from 

 behind forwards ; and others state that the blood is moved forwards 

 in the upper vessel and backwards in the lower. 



The arachnidans, which breathe by means of trachae, seem to have 

 a dorsal vessel only, without any ramifications. Those, on the 

 contrary, which possess branchial lungs, have a well developed cir- 

 culatory apparatus. The blood leaving the heart by the arteries is 

 distributed to all parts of the body ; having become venous it is 

 returned to the pulmonary branchiae by sinuses which supply the 

 place of veins, and having undergone arterialisation, it is returned 

 to the heart by the branchio-cardiac vessels, again to be propelled 

 through the arteries. 



In the higher Crustacea the heart consists of two sinuses and a 

 ventricle ; from the latter, in the stomopoda, six vessels arise which 

 distribute the blood to the eyes, liver, antennas, (fee. This fluid 

 is next returned to venous sinuses in the neighbourhood of the 

 branchiae, through passages without any apparent parietes ; the 

 arterialised blood is brought from the branchiae to the sinuses of the 

 heart by vessels named branchio-cardiacs, from which it is received 

 by the ventricle and sent into the arteries issuing from it, each of 

 which is provided with a valve at its origin. 



Cyclo-gangliata. — The greater number of the animals which 

 compose this class are aquatic, and enjoy a branchial respiration. 

 In the ascidiae the heart is very simple, consisting merely of a thin 

 membranous ventricle destitute of valves. The conchifera have 

 two auricles and a ventricle; the gasteropoda and pteropoda possess 

 a strong auricle and ventricle provided with valves. In the cepha- 

 lopoda there is an aortic heart, and two branchial hearts, or dilata- 

 tions ; in these animals, the blood having been carried to the system 

 by the arteries is returned by the veins to the branchial fringes ; in 

 some bivalves, however, a portion of it is sent direct to the heart 

 without passing through the respiratory organ. In the gastero- 

 podous and other mollusca the rectum passes through the ventricle. 



