STRUCTURE OF THE LEECH. 191 



36 on the eleventh. It is also worth noticing that the skin 

 of segments 9-11 is especially slimy, forming the so-called 

 clitellum or saddle, the secretion of which makes the cocoon 

 for the eggs. 



The skin is so closely connected with the connective and 

 muscular tissue lying beneath that little will be made of its 

 structure except by the microscopic examination of sections. 

 Most externally lies the cuticle — a product of the epidermis 

 —periodically shed as we have already noticed. In this 

 shedding some of the genuine epidermal cells are also thrown 

 off. These are somewhat hammer-like units with the heads 

 turned outwards, while the spaces between the thick handles 

 contain the pigment and the fine branches of blood-vessels. 

 As the latter come very near the surface a respiratory absorp- 

 tion of oxygen and outward passage of carbonic acid is 

 readily effected. Opening between the epidermal elements, 

 but really situated much deeper, are numerous long-necked, 

 bottle-shaped glandular cells, the contents of which form the 

 slimy stuff so abundant on the skin. Underneath the 

 epidermis there is much connective-tissue, and not a little 

 pigment, yellow and green, brown and black in colour. 



The muscular system^ by means of which the animal moves 

 so deftly, consists of long spindle-shaped cells arranged 

 externally in circular bands like the hoops of a barrel, inter- 

 nally 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 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. 



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-oesophageal) ganglia are connected 

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



