48 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



far from the hind end of the body. This aperture is termed 

 anus in the female, but cloacal opening in the male, because it is 

 common to the digestive and reproductive organs. The female 

 possesses a rounded genital aperture, situated in the mid-ventral 

 line about one-third of the way back. In both sexes there is a 

 minute excretory pore, placed mid-ventrally not far from the front 

 end. 



2. The body- wall (Fig. 1 5) consists of a firm, chitinous several- 

 layered cuticle, covering an epidermis, internal to which is a layer 

 of longitudinally directed muscle-cells. 



In a young specimen the cuticle is thin and the underlying 

 epidermis a distinctly cellular layer, but in the adult the formation 

 of a thick complex cuticle (c) seems to have caused degeneration 

 in the epidermis (ep\ which is then a thin granular stratum 

 through which numerous nuclei are scattered, and which is tra- 

 versed by gelatinous fibres. It is, however, much thickened mid- 

 dorsally, mid-ventrally, and, especially, laterally, to form the 

 dorsal (d.F), ventral (v.l), and lateral lines (1.1) to which reference 

 has already been made. Each of these is lined by a layer of 

 cuticle which is folded in so as to nearly divide the line into two. 



The muscle-layer is divided into four parts by the epidermal 

 lines, and is made up of flat muscle-cells. Each of these is about 

 i of an inch long, and consists of a transversely striated contractile 

 fibre, internal to which is a nucleated protoplasmic part produced 

 into a number of threads. The cell is directed longitudinally, 

 with its fibre closely attached to the epidermis, its protoplasm 

 projecting 'into the body-cavity, and its flat sides facing adjacent 

 muscle-cells. The thread-like internal processes rim to the dorsal 

 line in the upper half of the body, and in the lower half of the 

 body to the ventral line. 



3. The Digestive Organs (Fig. 15) consist of a narrow tube 

 which runs straight from mouth to anus or cloacal aperture. 

 This alimentary canal or gut is divided into three well-marked 

 regions (1) the fore-gut (stomodseum), (2) mid-gut (mesenteron), 

 and (3) hind-gut (proctodseum). The essential difference between 

 these is their mode of origin. In the course of development 

 (1) and (3) are formed as infoldings or in-pushings from the 

 exterior, and are therefore lined by ectoderm, while (2) is lined 

 by endoderm. 



The fore-gut consists of a thick-walled gullet (oesophagus) into 



