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filled with a circulating fluid containing numerous free- 

 cells (blood corpuscles). In the fluid there is a red sub- 

 stance, haemoglobin, which (through the skin circulation) 

 takes up oxygen from the outer air. The earthworm 

 breathes by its skin. The capillary walls are so extremely 

 thin, that transference of food and oxygen from the blood 

 to the body tissues and removal of waste are readily 

 effected. 



The principal vessels are the dorsal vessel, along the en- 

 tire length of the alimentary canal, the ventral vessel 

 below the alimentary canal, and, connecting these there 

 are short half-hoop-like vessels (two in each segment) of 

 which five enlarged pairs (in the region of the gullet) are 

 the contractile " hearts." Below the nerve cord there is 

 a sub- neural vessel and two lateral neural vessels. The 

 blood flows forward in the dorsal vessel, and downward 

 through the five pairs of rhythmically contractile " hearts " 

 into the ventral vessel in which the flow is backwards. 



2. Along the ventral body-wall there is a median ventral 

 nerve cord, the two halves of which diverge (in the third 

 segment) and form a ring round the pharynx. In each 

 segment the cord is somewhat swollen; each swelling 

 is an aggregation of nerve cells and is termed a ganglion 

 or nerve centre. The largest of these swellings is the doubje 

 ganglion of the nerve ring (the supra-pharyngeal or cerebral 

 ganglia) connected with which are the small peripheral 

 nerves of the sensitive prestomium (the lobe overarching 

 the mouth). From the ganglion in each segment three 

 pairs of nerves are given off on each side to the body. 



In the ectoderm there are scattered sensory cells. Each 

 has a fine sensitive hair which projects through the cuticle^ 

 and a delicate afferent or sensory fibril which is connected 

 with a nerve or ganglion cell. Impressions received from 

 the outside through the sensitive hairs are transmitted 

 to the ganglion cells which then convey impulses through 

 their efferent or motor fibrils to the muscles. A bunch of 

 sensory fibrils forms an afferent peripheral nerve ; a. 

 motor peripheral nerve is composed of efferent fibrils. 



