STRUCTURE 233 



distinctly divided into cells, covered by a chitinised cuticle ; it 

 bears cilia, sensory hairs, and peculiar scale-like processes, some- 

 times produced into long bristles. 



The cilia are chiefly arranged in two ventral bands, each 

 extending nearly the whole length of the body, and composed of 

 a series of transverse rows of single cilia ; along these bands the 

 hypoderm is thickened and more richly nucleated. The sides of 

 the head also bear numerous long cilia. 



The scales are hollow processes of the cuticle overlapping 

 from before backwards. A ventral row lies between the ciliary 

 bands ; two series of alternating dorsal rows lie on the back and 

 sides of the animal, and in the hirsute species it is these that 

 are produced backward into bristles. A single large scale, the 

 * frontal shield," protects the head above and behind, but does not 

 extend down to the ventral surface. On either side of the head 

 is a pair of flattened oval areas, the " lateral fields." From between 

 these on either side springs a tuft of motile sensory hairs. Two 

 pairs of similar tufts arise dorsally on the front margin of the frontal 

 shield, and a fourth pair spring from the ventral surface a little 

 behind the mouth. These hairs are distinguished from ordinary 

 cilia by their length, and their insertion on large nucleated cells 

 receiving nerves ; two pairs of similar hairs lie farther back on 

 the dorsal surface, one in the front of the neck, one near the 

 base of the pedal processes. 



The muscles lie some in the body -wall, and some traverse 

 the body-cavity ; only six pairs occur, simple, unstriated, and 

 longitudinal. There are neither transverse nor circular muscles. 



The alimentary canal is very simple and nearly straight 

 from mouth to anus ; it may be divided into pharynx, gullet, 

 stomach, and rectum. The mouth is circular, and looks forwards 

 and a little downwards. From the mouth opens the pharynx, 

 a short chitinous tube, capable of eversion by being pushed 

 forwards by the gullet ; it bears half-way down a circlet of 

 curved hooks, which open out when it is everted ; within these 

 are tooth-like thickenings. 



The oesophagus or gullet is thick and muscular, extending 

 through the whole of the neck of the animal ; its cavity, as well 

 as the opening from the pharynx, is triradiate like a leech -bite, 

 but can be dilated by the action of the muscular walls, inserted 

 into a firm external cuticle ; the internal wall is also cuticulised, 



