ARTHROPODA. 277 



cuticle is separated from the epidermis, rupturing along some 

 line of weakness, and allowing the escape of the animal. This 

 moulting extends not only to the minutest of the external 

 organs, but to the stomodseum and proctodseum as well. A 

 new cuticle begins to be secreted at once, but this " soft- 

 shelled " condition is one of great danger and helplessness to 

 the animal. The process besides is exhausting, and to these 

 facts we may attribute, in part at least, the small size of most 

 arthropods. The cuticula is laid down very thinly at the joints. 

 Thus is secured the flexibility necessary in locomotion. 



312. The Muscles are well developed, and many of the 

 arthropods are very powerful in proportion to their size. The 

 circular muscles characteristic of the annulata are lacking in 

 the arthropods. The chief body muscles are the longitudinal 

 which cause the flexion and extension of the segments. There 

 are in addition the muscles by which the appendages are 

 moved. These fibres are of the cross-striate type. Less 

 massive groups of fibres are found in the walls of portions of 

 the digestive tract. 



313. The Digestive Organs. The alimentary tube is 

 typically rather complex. It commences with a mouth which 

 is usually supplied with three or more pairs of external 

 appendages assisting in the capture, transfer, and preparation 

 of food. This is followed by an oesophagus either with or 

 without a crop; a stomach frequently consisting of several 

 regions: viz. (a) a proventriculns or gizzard, and (&) a 

 ventriculus or stomach proper ; an intestine which is not always 

 clearly marked off from the stomach ; and a posterior open- 

 ing, the anus. The development of the gut shows both 

 stomodseum and proctodreum (see 90). The former is often 

 very extensive, embracing even the proventriculus, in which 

 chitinous grinding plates may occur (cray-fish, cockroach). 

 The " salivary " glands when present Open into the oesophagus 

 or mouth cavity. Into the mesenteron important digestive 

 glands may open, as the pyloric caeca (many insects), or liver 



