546 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The outermost layer of the body is a dense chitinous 

 cuticula, usually divisible into several layers. These layers 

 may be fibrillated, tlie direction of the fibrillation being dif- 

 ferent in tlie successive layers. Cilia are found neither on the 

 surface, nor elsewhere, at any period of life. The mouth is 

 situated at one extremity of the body, the anus at, or near, 

 the other end. The first portion of the alimentary canal is 

 a thick-walled pharynx, lined by a continuation of the chiti- 

 nous layer of the integument, which may be raised up into 

 ridges or tooth-like prominences. Transverse fibre?, appar- 

 ently of a muscular nature, radiate from the lining of the 

 pharynx through its thick wall, and probably serve to dilate 

 its cavity. A straight and simple tubular alimentary canal, 

 without any distinction into stomach and intestine, extends 

 through the axis of the body, a narrow oesophageal portion 

 usually connecting it with the pharynx. 



The endoderm, or wall of the alimentary canal, consists 

 of a single layer of cells, disposed in few or many longitu- 

 dhial series ; and lined, both internally and externally, by a 

 cuticular layer. On each side, the intestine is fixed through 

 its whole length to the " lateral area," to be described below. 

 The cuticle, which lines the inner faces of the endodermal 

 cells, and circumscribes the digestive cavity, appears, on verti- 

 cal section, to be divided into rods, which are possibly merely 

 the intervals of minute vertical pores. In some cases, muscu- 

 lar fibres invest the posterior portion of the intestine. 



Beneath the layers of the chitinous cuticle there is a 

 proper integument, or ectoderm, internal to which again is 

 a single layer of longitudinally-disposed muscles, which may 

 or may not be divided into distinct series of "muscle-cells." 

 The space between these and the outer face of the intestine 

 is occupied by a spongy or fibrous substance, which must 

 probably be regarded as a kind of connective tissue. The 

 muscles and this tissue, taken together, constitute the meso- 

 derm. 



In the typical N'ematoidea^ the muscular layer does not 

 form a complete investment of the body, but is interrupted 

 along four equidistant longitudinal lines. One of them is 

 termed dorsal, the opposite ventral, and both these are very 

 narrow. The other two are much broader, and are termed 

 the lateral areas. They often (Fig. 156) present two or more 

 series of conspicuous nuclei, and each is traversed by a canal 

 with well-defined contractile walls and clear contents. Op- 

 posite the junction of the oesophageal with the gastric por- 



