ARTHROPODA 28 I 



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. Chitin is not deposited in the cuticula at the joints, 

 thus securing the flexibility necessary in locomotion. 



319. Muscles and Locomotion. — The muscles are well 

 developed, and many of the arthropods are very powerful in 

 proportion to their size. The circular muscles characteristic 

 of the body wall of Annelida 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. 



The paired, segmented appendages are primarily organs of 

 locomotion. They are variously used for crawling, walking, 

 jumping, and swimming. Many have wings, and the quality 

 of the nervous and muscular control of these can be realized from 

 the fact that the honey bee can make some 326-435 wing strokes 

 per second and the house fly 330 per second. Some forms, as the 

 crayfish, have the power of using segmented portions of the 

 body in a powerful backward swimming stroke. Some types 

 have the power of skating round on the surface of water. 

 Locomotion in the phylum is remarkably varjed and efficient. 



320. Digestion and 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 esophagus either with or 

 without a crop; a stomach frequently consisting of several 

 regions: viz. (a) a proventriculus or gizzard in which food is 

 further broken to pieces physically, and (6) a ventriculus or 

 stomach proper in which chemical digestioii occurs ; an intestine 

 which is not always clearly marked off from the stomach ; and a 

 posterior opening, — the anus. The development of the gut shows 

 both stomodseum and proctodeum (see §93). The former is 

 often very extensive, — embracing even the proventriculus, in 



