236^ LECTURE XI. 



they may be deemed homologous with the tracheal respiratory organs 

 of the higher Articulata ; but in fanction they seem to be reduced to 

 supplying the skin with its abundant mucous secretion, and the ova 

 with their cocoon-like coverings at the season of generation.* 



Morren has minutely described the circulating system of the earth- 

 worm; in a species of which (^Lumbricus variegatus) Bonnet j" saw 

 the red blood propelled forward by the systole and diastole of the 

 dorsal vessel towards the head, and noticed its accelerated course 

 near that part. 



In the tubicolar anellids, according to Dr. M. Edwards |, the 

 dorsal contractile artery is unusually short. In the Terebella it re- 

 ceives numerous veins from the intestine, and a large accession of 

 the circulating fluid from two wide transverse venous trunks, which 

 encircle the commencement of the intestine, and which receive recur- 

 rent v^ins from the oesophagus, also a small vein from the integu- 

 ments of the back. This short dorsal vessel is, in fact, the general 

 receptacle of the venous system, and, by its function, it represents a 

 pulmonic heart. It transmits the venous blood almost exclusively to 

 the cephalic branchiae by three pairs of branchial arteries, which 

 arise from its anterior extremity. The oxygenated blood is returned 

 by the branchial veins to a large ventral trunk, situated immediately 

 above the ganglionic nervous chord. This vessel, which has the 

 function of a systemic heart or aorta, supplies a pair of transverse 

 branches to each ring of the body, which distribute filaments to the 

 integuments and the feet, and then ascends to supply the intestine, 

 where, with the absorbent veins of that canal, it returns again into 

 the dorsal vessel. In some other species of Terebella, as the Ter. 

 conchilega, the lateral branches of the ventral trunk do not ascend in 



* Dr. 'Williams (CXC. p. 257.) affirms that " spermatozoa can always be dis- 

 covered in the interior of these sacs j" and states that they are " true vesiculas 

 seminales." I have never been so fortunate as to detect spermatozoa in the mucus- 

 eacs of young leeches, in which those parts have been relatively as fully developed 

 as in the full-grown leeches fit for copulation. The looped glands {g) are described 

 by the same writer to be ovai-ia, " ovarian utricles," the glandular granular par- 

 ticles, enclosed in their walls, which are fignred and described by Brandt and 

 Ratzeburg (CLXXXV. " Drusensubstanz," p. 251, tab. xxix. A. fig. 8), having 

 apparently been mistaken for ova. Dr. Williams further describes a minute duct 

 as extending outwards, crossing beneath tlic longitudinal sperm-duct, and " be- 

 coming united to the base of the ovarian utricle," p. 254 (looped mucus-gland, g). 

 Although my dissections have enabled me to distinguish the small blood-vessels 

 uniting the testes to a neighbouring mucus-duet, I have failed to make out the 

 spcrm-dnct so described. 



f Observations sur les Vers, CEu^Tes, i. p. 193., 1779. 



X Sur la Circulation dans les Anellidcs, Annales dcs Sciences, Nat. X. p. 121. 



