188 KEPORT— 1851. 



The sub-ganglionic trunk in this worm exceeds the dorsal in calibre; it 

 re-circulates throughout the system the contents ol' the dorsal trunk ; lateral 

 branches, slightly coiled and lengthened, a provision against injury during 

 the vermiculations of the body, are detached at each ring, to the feet and 

 intestine. Those for the former penetrate at the roots of these appendages 

 and reach the cutaneous surface, whereon a complex network of capillary 

 vessels is formed, veiled from the exterior only by a layer of epithelium. 

 This plexus is the true respiratory organ of the Nereid (see fig. 13, a). This 

 plexiform subdivision of the vessels is not seen in many worms ; it is a forma- 

 tion almost peculiar to the Nereids. In the neighbourhood of these respiratory 

 plexuses, artfully arranged, a system of vibratile cilia is provided, without 

 which the great function devolving on these vessels were incompletely dis- 

 charged. The intestine is embraced in a framework of four longitudinal 

 vessels, between which a glandular capillary system intervenes, which pro- 

 vides the digestive secretions. 



In the Nereids, then, no heart-like centre to the circulation exists. The 

 great dorsal, the reservoir of the centripetal streams of the body, may be 

 likened to a right ventricle (the lungs cut off), and the great ventral to a 

 left ventricle. The duty of the former is to collect the refluent blood of the 

 system, of the latter to circulate it again. 



A slight modification in this system occurs in Nephthys Hombergii, a dorsi- 

 branchiate Annelid allied to the Nereids, and common on our coasts. A 

 strong proboscis, enclosed in an oesophagus of corresponding strength, by 

 its constant motions would, in this worm and in this situation, endanger the 

 safety of lateral vessels. The position of these branches is accordingly 

 thrown back as far as the commencement of the intestinal division of the 

 alimentary tube. The crowding of the branchial veins upon this point of 

 the vessel imparts to the latter an augmented diameter or a heart-like form. 

 The vessel then creeps along the dorsal surface of the proboscidian oesopha- 

 gus, neither giving nor receiving branches, as far as t!ie occipital segment, 

 where it divides into two branches which descend on either side to the ven- 

 tral trunk, while a i'ew small twigs proceed forward to supply the tentacles. 

 In this worm a distinct branchial organ is provided, which is situated at the 

 inferior base of the superior foot. The branchial veins stretch across the 

 peritoneal space and empty themselves directly into the great dorsal vessel. 

 As in the Nereids, the afferent blood-vessels of the branchiae are derived 

 from the great ventral ; the branches corresponding in number with the 

 segmental divisions of the body. It remains to note a peculiarity of the 

 ventral or sub-ganglionic system in Nephthys, which Milne-Edwards was the 

 first to recognise. The vessel, which is a single trunk in all other Annelids, 

 is double in this. The parallel trunks, however, communicate freely here and 

 there by cross branches. It appears then that the lateral series of vessels of 

 one side destined for the branchiae are independent of those of the other. 

 This conformation, so remarkable, has a meaning. It is a beautiful provi- 

 sion against the consequences of injury, to which the habits of this worm 

 render it obnoxious. 



The resemblance is most striking between the circulating system of Nais 

 Jiliformis and that ofArenicola Piscatormn (fig. 10). From the dimensions of 

 this last worm, it is easily dissected ; the older anatomists had discovered the 

 existence of a heart-like centre. Hunter, SirE.Home and Lamarck, have each 

 described the blood- system of this vulgar worm. But it was reserved to 

 Prof. M. Edwards to unravel its details with a demonstrative accuracy worthy 

 of modern science. Nothing remains to be added to the account given by 

 this naturalist. The blood-system is more centralized in this worm than in 



