ON THE BRITISH ANNELIDA. 177 



one ventral, whicli may be distinguished as the sub-intestinal ; and two lateral. 

 These several trunks are joined together by circularty disposed branches, 

 bearing a dense, glandular, capillary system. In the inferior intestinal system 

 the general movement of the blood is from before backwards, in the circular 

 branches from the ventral towards the dorsal trunk. 



5th. In Arenicola, Nais, Lumhriciis, Hirudo, the dorso-intestiiial trunk 

 sends off the afferent branchial vessels, and tliese latter return into the great 

 dorsal trunk. In these species the former vessel therefore discharges the func- 

 tions of a pulmonary artery or branchial heart. 



6th. In the Terebellce and Serpulce, which are cephalobranchiate, the ante- 

 rior extremity of the great dorsal trunk enlarges fusiformly and propels the 

 blood directly into the branchial appendages. In these genera tlierefore 

 this vessel becomes the branchial heart ; and the great ventral trunk, into 

 which the efferent branchial vessels empty themselves, becomes the systemic 

 aorta. 



7th. In all cases, without exception, the three inferior intestinal trunks 

 carry arterial blood, and in nearly all species, the dorso- intestinal venous. 



8th. In Arenicola, Nais, and the Borlasice and Liniadce, there exists a di- 

 stinct heart. 



9th. In all other species the main vessels, more or less modified in different 

 species, constitute the propulsive centres. 



To these general statements, which in the Annelida express the main laws 

 of the circulating system, no real exception occurs. 



In different species different portions of this system receive augmented de- 

 velopment, in accordance, first, with the method of the respiratory process, the 

 position of the branchiae, and the degree of muscular mobility conferred on 

 different parts of the body. In those species in which locomotion is accom- 

 plished by alternate and extreme elongation and contraction of the body, the 

 vessels are in all parts remarkably coiled and convoluted. This feature is 

 exemplified most perfectly in Naisfiliformis. If a part of the body only be 

 endowed with this vermiform mobility, the convoluted character of the blood- 

 vessel is limited to such part ; and this embraces most frequently the region 

 of the oesophagus. 



In the Leech the circulating system is more highly developed than in any 

 other Annelid. The presence or absence of a heart-like centre to this system, 

 is by no means in this class of animals the true criterion of the degree of its 

 evolution. The amount of blood relatively to the size of the body, the degree 

 of capillary subdivision which occurs on the periphery of the blood-system, 

 and the proportion of the latter to the peritoneal fluid, form more correct 

 indications. In the Leech there exists no free space between the intes- 

 tine and integument ; to this anatomical fact the highest interest will be sub- 

 sequently shown to belong when explaining the mechanism of respiration in 

 this Annelid. Here the chylous fluid, which, as formerly shown in nearly all 

 other Annelids, occupies the general cavity of the body, like a cylindrical 

 fluid stratum, separating the intestine from the integument, is transferred into 

 the interior of the lateral diverticula of the stomach. The peritoneal chamber 

 being no longer required, is obliterated by the adhesion of the intestine to 

 the integument ; the union of these parts is effected through the medium of 

 a dense, spongy layer of capillary blood-vessels, the contents of which are 

 exposed internally to the influence of the fluid contained in the digestive 

 caeca, and externally to that of the circumfluent element ; hence the mecha- 

 nism of the respiratory process, and the power enjoyed by this and other 

 abranchiate Annelids of dispensing with all external breathing appendages. 

 While, however, the peripheral segment of the vascular system in the Leech 



1851. N 



