HIKUDO MEDICINALIS. 217 



and, when once pointed out, will be found sufficient for the guidance of 

 the most superficial observer. The body is always considerably elon- 

 gated, and composed of a succession of rings or segments that, with the 

 exception of the first and last, scarcely differ from each other except in 

 size. Each ring is generally found to be furnished with a set of short 

 spines or setas, calculated to assist in locomotion ; but in no instance are 

 these animals provided with articulated legs. The first segment of the 

 body, which may be called the head, contains the mouth, sometimes 

 provided with a formidable apparatus of jaws ; it is also generally 

 furnished with eyes, and variously- shaped tentacula, apparently instru- 

 ments of touch. The last segment also not unfrequently presents seti- 

 form appendages, and occasionally a prehensile sucker, used as an organ 

 of progression. 



(568.) Their blood is sometimes remarkable for its red colour, and 

 circulates in a double system of arteries and veins ; respiration is effected 

 either in the general cavity of the body, or by means of arborescent 

 tufts appended to various parts of their external surface ; moreover 

 they are almost all hermaphrodite, and generally require the congress 

 of two individuals for mutual impregnation. 



(569.) ABRANCHIATA. This order comprises two distinct tribes, that 

 differ widely in their habits and external appearance : the first com- 

 prehends the LEECHES (Annelida suctoria), distinguished by the exist- 

 ence of a prehensile sucker situated at each extremity ; while in the 

 second, instruments of attachment are totally wanting, the only external 

 appendages to the body being a number of minute and almost impercep- 

 tible bristles, which project from the different segments and assist in 

 progression : such are the EARTHWORMS, &c. (Annelida terricola). 



(570.) The common Leech (Hirudo medicinalis) affords the most 

 interesting example of a suctorial Annelid. The outward form of one 

 of these animals is familiar to every one, and their general habits too 

 well known to require more than a brief notice. The body is very 

 extensible, and divided by a great number of transverse lines into 

 numerous rings, apparent in the contracted state of the animal, but 

 nearly imperceptible when the body is elongated. The skin is soft, 

 being merely a thin cuticular pellicle separable by maceration ; and the 

 surface is lubricated by a copious secretion of mucus. Beneath the 

 cuticle is a layer of coloured pigment, upon which the colours of the 

 animal depend ; but the cutis, or true skin, is so intimately connected 

 with the muscular integument of the body, that its existence as a 

 distinct tunic is scarcely demonstrable. The muscular coverings or 

 walls of the body, which form a kind of contractile bag enclosing the 

 viscera, are found, upon accurate dissection, to consist of three distinct 

 strata of fibres running in different directions. The outer layer is 

 composed of circular bands, passing transversely ; in the second the 

 fibres assume a spiral arrangement, decussating each other ; while the 



