ANNELIDA. 



191 



sen ted in the annexed figure, there is a remarkable provision made 

 for closing the entrance Fig. 78. 



of the tube when the 

 animal retires within its 

 cavity. On each side 

 of the mouth is a fleshy 

 filament resembling a 

 tentacle ; but one of 

 these, sometimes the 

 right and sometimes 

 the left, is found to be 

 considerably prolonged, 

 and expanded into a 

 funnel-shaped opercu- 

 lum, which accurately 

 fits the orifice of the 

 shell, and thus forms 

 a kind of door, well 

 adapted to prevent in- 

 trusion or annoyance 

 from external enemies. 



(234.) Abranchia. The common Leech (Hirudo medicinalis) 

 affords the most interesting example of a suctorial Annelide. 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 very 

 brief notice. The body is very extensible, and divided by a great 

 number of transverse lines into numerous rings, extremely 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 lubri- 

 cated 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 covering or walls of 

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

 viscera, is 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 internal layer is made up of longitudinal 



