3 o COMPARATIVE ANATOMY 



of the blood stream, differs considerably in different species of 

 oligochcete worms, so that the statements of one author fre- 

 quently seem to stand in contradiction to those of another who 

 has worked on a different species. The account given above 

 is probably nearly correct for L. herculeus. Itjvill be observed 

 that the blood of the sub-intestinal vessel must pass either 

 through the body-wall or through the nephridia before it finds 

 its way back to the supra-intestinal trunk, so that in the one 

 case it is oxygenated, in the other case it is purged of 

 excretory nitrogenous material. In the anterior region of the 

 body, where there are no commissural vessels, the oxygenated 

 blood from the body-wall is carried back to the intestino-tegu- 

 mentary vessels, and then distributed to the pharynx and 

 anterior segments. 



It is beyond the scope of the present work to enter into a 

 detailed account of the histology of the different organs of the 

 earthworm, but the general relations of the organs and the 

 general histology of the tissues as visible in transverse section 

 are represented in fig. 7. Externally the body is invested by a 

 cuticle, which may easily be stripped off a worm which has 

 been left some time in water. It is perforated by numerous 

 minute pores, the orifices of unicellular glands of the epi- 

 dermis, and viewed from above it is seen to be traversed by 

 two sets of fine lines or striae, which cause the iridescent hues 

 mentioned above. Beneath the cuticle is the epidermis 

 formed of two layers of cells. The innermost layer consists 

 of very small cells, the inner broad ends of which lie on a 

 basement membrane, their outer ends being pointed and 

 wedged in among the cells of the outer layer. The outer 

 layer consists of gland-cells, interstitial cells, and sense cells. 

 The gland cells are pear-shaped, the narrower end directed 

 inwards, and the swollen outer part filled with granules. The 

 interstitial cells are much smaller than the gland cells, and 

 may be described as modified columnar epithelial cells. 

 Amongst them are many rod-shaped cells, not .differing much 

 in appearance from ordinary interstitial cells, but having 

 their inner ends produced into fine fibrils, one of which is 

 continued into a nerve trunk as a sensory or afferent nerve 

 fibril. In the anterior segments of the body groups of sense 

 cells furnished with fine processes at their outer extremities 

 are found, chiefly in proximity to the chsetae. The modified 



