100 ESSENTIALS OF ZOOLOGY 



the divisions between the cells account at the same time 

 for the prismatic appearance and the pores. 



The integument also forms the skeleton, and in relation 

 to the muscles it is produced inwards to form so-called tendons 

 (apodemata). Such internal projections of the cuticle form 

 a complicated endophragmal skeleton on each side of the floor 

 of the thorax, a skeleton which serves for the attachment of 

 the prominent muscles of the legs and the flexor muscles of 

 the abdomen ; it also forms a canal for the ventral nerve 

 cord. 



Muscular System. The muscles of the body and the limbs 

 are striated and white in appearance. They are especially 

 well developed in the abdomen and the chelae. They are 

 attached to the basement membrane, and the thin ectoderm 

 therefore intervenes between them and the cuticle. As has 

 already been pointed out, internal foldings of the cuticle are 

 developed in relation to many of the muscles, and in some 

 cases, as for example the adductor muscle of the mandible 

 and the strong muscles of the chelae, the internal process 

 is long and tendon-like. In the abdomen the muscles are 

 symmetrically disposed on each side, and consist of a dorsal 

 pair originating from the side wall of the thorax and dividing 

 into a series of bundles which are inserted in succession into 

 the various terga, and of a stronger ventral pair originating 

 from the upper part of the endophragmal skeleton, which 

 similarly divide to be inserted into the sterna. The segments 

 of the abdomen are articulated by joints at the sides. The 

 dorsal pair, therefore, act as extensors and the ventral pair 

 as flexors. The muscles of the appendages pass from their 

 origin on the inside of the segment in each case to be inserted 

 into the edge of the next segment. There are important 

 muscles also in relation to the stomach. 



Alimentary Canal. This consists of a short oesophagus, 

 a large two-chambered stomach and a straight intestine. 

 The inner layer secretes a lining of chitin continuous with the 

 external cuticle at the mouth and anus, except in the small 

 anterior portion of the intestine into which the bile ducts 

 open. Thus the digestive tube may be divided into a fore- 

 gut or stomodeum, including the oesophagus and stomach, 



