216 SEGMENTED WORMS OR ANNELIDA. 



cells, the contents of which form the mucus so abundant on 

 the skin. Underneath the epidermis there is much con- 

 nective tissue, and not a little pigment, yellow and green, 

 brown and black in colour. 



The Muscular System. 



The muscular system consists of spindle shaped cells 

 arranged externally in circular bands like the hoops of a 

 barrel, internally in longitudinal strands like staves. Besides 

 these there are numerous muscle bundles running diagonally 

 through the body, or from dorsal to ventral surface, and 

 there are other muscles associated with the lips, tooth plates, 

 and pharynx. 



The Body Cavity. 



The body cavity is almost quite obliterated in the adult 

 leech, where the predominant connective tissue has filled up 

 nearly every chink and crevice. It is to be seen in the 

 embryo, and its remnants may be detected here and there 

 in the adult. The virtual absence of the body cavity, and 

 the spongy compactness of the whole animal, make the 

 leech a tedious subject to dissect. 



Nervous System. 



The nervous system mainly consists of a pair of dorsal 

 ganglia lying above the pharynx, and of a double nerve cord 

 with twenty-three ganglia lying along the middle ventral line. 

 The dorsal (or supra cesophageal) ganglia are connected with 

 the most anterior (or sub-cesophageal) pair on the ventral 

 chain, by a narrow nerve ring surrounding the beginning of 

 the gut. From the dorsal centres nerves proceed to the 

 " eyes " and anterior sense spots, from the ventral centres 

 the general body is innervated, and from the beginning of 

 the ventral chain special nerves supply the alimentary canal, 

 forming what is called a visceral system. 



The Sense Organs. 



The sense organs of the leech are ten so-called "eyes," besides 

 numerous sense spots usually occurring on every fifth skin ring. The 

 eyes are arranged round the edge of the mouth, and look like little 

 black spots. Microscopic examination shows them to be definite cups, 

 surrounded by connective tissue with black pigment, and containing 

 clear strongly refracting cells, each in connection with a fibre of the 

 optic nerve. 



