92 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The motor nerve-fibres which supply muscle come into very close 

 relation with its fibres. The primitive-sheath becomes continuous 

 with the sarcolemma, and the axis-cylinder passes into a granular, 

 nucleated end-plate, resting upon the muscle substance. 



4. Digestive Organs (Fig. 28, A). The forceps and clawed 

 walking-legs are accessory to digestion, and the six pairs of jaws 

 overlapping one another outside the mouth are still more closely 

 connected with the same function. But all these are more or less 

 modified appendages, and outside the alimentary canal. This is 

 a tube which runs straight from mouth to anus and consists of a 

 capacious fore-gut (gullet and stomach), a very small mid-gut 

 into which a large digestive gland opens, and an elongated tubular 

 hind-gut (intestine). 



The ventral mouth (m), which possesses anterior and posterior 

 lips (I and mt), leads into the fore-gut (stomodseum). This consists 

 of a short, wide, upwardly-directed gullet (oesophagus) (05), and a 

 large sac, the stomach, into which this opens. The stomach fills 

 most of the cavity in the head, and is divided into a larger 

 cardiac (ca) part in front and a smaller pyloric (py) part behind, 

 the two being separated by a deep constriction. The fore-gut is 

 an ingrowth from the exterior, and it is not, therefore, surprising 

 to find it lined by a firm cuticle, continuous at the mouth with 

 the exoskeleton. The cuticular lining of the stomach is locally 

 thickened and calcified into hard plates and bars (sclerites) which 

 form a chewing-apparatus or gastric mill, situated for the most 

 part in the posterior part of the cardiac division. Numerous 

 projecting setae constitute a strainer in the pyloric part of the 

 c stomach. The sclerites of the gastric mill 



are united together into an elastic hexagonal 

 framework with anterior and posterior sides 

 connected by a jointed rod. These can be 

 diagrammatically represented as in annexed 

 P plan, the arrow pointing to the front. 



A broad cardiac sclerite (c) lies transversely in the dorsal wall of 

 the cardiac division. Attached to the middle of this is a small 

 uro-cardiac sclerite (u), directed downwards and backwards in the 

 front wall of the constriction separating the two regions of the 

 stomach. These two sclerites form a T-shaped arrangement. 

 A second and somewhat similar J_-shaped combination is formed 

 by a pyloric sclerite Q?) placed across the dorsal wall of the pyloric 



