Arthropoda. 



185 



a certain thinness, so ttat movement can take place at these points. 

 All Arthropods moult* periodically, at least, as long as growth 

 continues; the cuticle loosens from the underlying tissue, breaks 

 at some point, and is cast ofE as a whole {i.e., the animal creeps 

 out of it) after the epidermis has secreted a new cuticle. This is 

 thin and soft at first, but becomes thick and hard later. Such 

 periodic ecdyses are indispensable 

 for growth, for the stiff, un- 

 yielding cuticle allows only of very 

 slight increase in the size of the 

 body. The growth of the animal 

 would therefore cease, if the sur- 

 rounding case were not now and 

 again thrown off and replaced by 

 a new and roomier one. Upon 

 the body, there are larger or 

 smaller tracts of setae, evagina- 

 tions of the cuticle, each contaiaing 

 a process of the soft epidermis; 

 the cuticle at the base of the hair 

 is thinner, so that it can move 

 about. The cuticle consists of 

 chit in, an organic substance^ of 

 a horny appearance, chemically 

 however, quite different from horn. 

 Lime salts, principally carbonate 

 of lime, are often deposited in the 

 chitin, especially in the Crustacea. 

 The skin is never ciliate among 



the Arthropoda, nor indeed is any other organ; in fact ciliated 

 cells are entirely absent. 



The muscular system is closely connected with the skin ; 

 the formation of a segmented exoskeleton, however, necessitates 

 important deviations, from the Annelid type. Instead of a 

 continuous musculature beneath the skin, there is usually a large 

 number of separate muscles passing from one segment to another, 

 and attached by their extremities to the inner side of the skin : 

 by their contraction the segments of the body, as also the 

 joints of the appendages, move upon one another. The muscles 

 are often connected by the so-called tendons, which, in the 

 Arthropods, always consists of invaginations of the cuticle, sur- 

 rounded of course by a corresponding invagination of the epidermis 



mf 



Pig. 146. Section ttrough a hair and 

 the adjacent sTdn of an Arthropod ; dia- 

 grammatic, c cuticle, d thin part at the exit 

 of the hair h ; ep epidermie. — Orig. 



* In many (all ?) Annelids (e.g.. Leeches) and in NematodeSj a similar eodysis 

 occurs. 



