196 



THE CRAYFISH. PHYLUM ARTHROPODA 



somite. The tergum, sternum, and pleura of each somite form a 

 continuous ring. The hmbs are jointed to the hinder side of the 

 sternum near its outer ends, and the part of the sternum between 

 each Hmb and the adjoining pleuron is sometimes called an 

 epimeron. The terga overlap one another from before backwards 

 and slide over one another as the abdomen is straightened and 

 bent, the armour of each somite being joined to that of the next 

 by thin cuticle, which allows of movement. In the cephalothorax 

 the terga are fused to form a shield or carapace. This is prolonged 



in front into a beak-hke rostrum and 

 is crossed by a furrow, which is called 

 the cervical groove because it is sup- 

 posed to mark the separation of two 

 regions known as the head and thorax. 

 At each side of the body a fold of the 

 carapace overhangs as a lean-to roof, 

 the gill cover or branchiostegite, w^hich 

 encloses between itself and the side 

 of the bod}^ a chamber in which the 

 gills lie. Behind the cervical groove a 

 branchiocardiac groove on each side 

 marks off the branchiostegite from a 

 median cardiac region which roofs the 

 thorax. The cuticle of the inside of the 

 branchiostegite and part of the side of 



Fig. 1^2. — View of a crayfish ,, , , ■, ii -x j.v^- r\ 



from above.— After Huxley, the body underneath it are thm. On 



Note cervical groove, cardiac region of thc VCUtral Sidc of thc CCphalothoraX 

 carapace, rostrum, and tail fan. •,,•-■ r -i • -i i j_i 



the limbs of each pair are close together, 

 but small sterna lie between them. The head, is of course, the 

 region which contains the mouth and the principal sense organs. 

 The mouth is placed on the ventral surface at some distance 

 from the front end, and in front of it the sternal surface slopes 

 upwards to the rostrum. At the sides of the latter, upon a pair 

 of short, movable stalks, are placed the eyes, and below these 

 stand two pairs of feelers or antennae. 



LIMBS 



The limbs or appendages number nineteen pairs, without 

 counting the eyes, which are by some authorities reckoned as 

 limbs. We shall not take this view, but as there is evidence in 



