448 



ZOOLOGY 



SECT. 



The whole alimentary canal is lined internally by a cuticle, which 

 is thicker in the gizzard than elsewhere, and by a single layer 

 of ciliated columnar epithelial cells, the enteric epithelium. Some 

 of these cells, more granular than the others, grouped in certain 

 regions more particularly along the typhlosole are of the nature 

 of unicellular digestive glands, secreting a digestive fluid. Others 

 seem to be specially concerned in the absorption of the digested 

 food. External to this is a layer of connective tissue, between 

 which and the external covering of yellow cells are muscular fibres, 

 of which there are two layers, an external longitudinal and an 

 internal circular. These layers are greatly thickened in the walls 

 of the pharynx and of the gizzard. 



The Earthworm, like Nereis, has a well-developed vascular 

 system consisting of blood-vessels with well-defined walls. The 



blood is bright red, the colour 

 being due to the same colouring 

 matter, viz. hcemoglobin, as in 

 the case of the blood of the 

 higher animals, occurring, how- 

 ever, not in corpuscles, but in 

 the liquid part or plasma ; 

 corpuscles are present, but they 

 ,,, are colourless. The main trunks 

 are the dorsal, the ventral, 

 the sub-neural, the two lateral 

 neural, and a series of trans- 

 verse branches. The dorsal vessel 

 (Fig. 365, dors, v) runs along 

 the middle of the dorsal surface 

 between the body-wall and the 

 intestine ; it is readily visible 

 FIG. 36?. Lumbricus. Anterior portion of shining through the former in the 



% living worm. The ventral vessel 

 PWSt ' prostomium ' (vent.v) lies below the alimentary 

 canal, the sub-neural below this 

 again under the nerve-cord ; the lateral neural lie on either side of 

 the nerve-cord. The transverse branches correspond in number to the 

 segments ; they run round from the dorsal vessel to the ventral, 

 giving off branches in their course. Five of them, viz. those in 

 the seventh to the eleventh segments inclusively, are dilated and 

 pulsate rhythmically ; these have the function of driving the 

 blood through the system of vessels, and are hence frequently 

 termed the " hearts." The walls of the principal vessels are 

 contractile, and assist in bringing about the movement of the 

 blood, which is propelled in such a way as to run forwards in the 

 dorsal vessel and backwards in the ventral, its direction of move- 



