THE CRAYFISH 115 



tries to lift up the posterior edge of the carapace one finds that 

 its median or cardiac part is firmly attached to the body-wall 

 beneath, and is in fact the thickened and calcified cuticle of 

 the fused thoracic segments. It differs, therefore, from the 

 shield of Apus, which belongs to the head region, and simply 

 overlies without being attached to the thoracic segments. 



The sides of the crayfish's carapace, however, are separated 

 by a space from the body-wall, and can easily be removed by 

 cutting along the outside of the branchio-cardiac groove and 

 continuing the cut forwards on the outside of the anterior 

 continuation of the cervical groove. The piece thus removed 

 is the branchio-stegite ; it is a lateral fold of the cuticle of the 

 cephalothorax ending behind, below, and in front in a free 

 margin. The space between it and the body wall is the 

 branchial chamber, in which may be seen the gills or branchiae 

 lying close against the body-wall. Their structure and relations 

 will be described later. 



Though the carapace is not jointed, examination of the 

 ventral surface of the cephalothorax shows that between the 

 bases of every pair of limbs there is a skeletal piece corre- 

 sponding in position to the sterna of the abdominal segments. 

 Every such piece is the sternum of a segment. The sternum 

 of the last pair of thoracic limbs is fairly wide and movable. 

 (In the lobster it is immovably fused to the one in front of 

 it.) As one proceeds forwards, the sterna of the thoracic 

 limbs become narrower, until those of the anterior thoracic and 

 oral limbs become very narrow and keel like. The sterna are 

 immovably fused together and lie in a straight line. In 

 front of the mouth the sternum of the segment bearing the 

 second antennae is broad and slopes upwards, that of the 

 segment bearing the first antenna is narrow and set nearly at 

 right angles to the sternum behind it so that it looks directly 

 forwards. This disposition of the two anterior sterna gives 

 rise to the cephalic flexure, conspicuous in a lateral view of 

 the animal. Noticing that, as in Apus, the mouth is placed 

 on the ventral surface and well behind the anterior extremity 

 of the animal, while the anus opens on the under surface of 

 the anterior joint of the telson, we may pass to a consideration 

 of the appendages. 



The eyes of the crayfish are situated in cavities at the base of 

 the rostrum, and being placed upon short movable stalks they 



