AKTHKOPODA 



177 



large enough to show the distinctive features without dif- 

 ficulty. It moves through the water by contracting the 

 tail or abdomen. This part of the animal is sharply 

 segmented and the plates of which it is composed slip 



Pig. 98.— Anatomy of crayfish. A, dorsal surface removed j B, scheme of circula- 

 tion ; C, viscera removed, showing green gland and nervous system, a, anus; 

 aa, hepatic artery ; ae, antenna ; ai, antennula, also sternal artery ; am, muscles 

 of stomach; ao, ophthalmic artery; ap, abdominal artery; av, ventral artery; 

 bl, urinary bladder ; br, gill arteries ; c, oesophageal commissures ; gd, green 

 gland ; gn, brain, central ganglia ; h, heart ; hd, intestine ; k, mandibular mus- 

 cles ; Ll\ liver and its duct ; m, stomach ; o, otoc\st ; as, oesophagus ; on, optic 

 nerve ; pc, pericardium ; sgn, sympathetic nerve ; t,t', testis or spermary ; v, ven- 

 tral blood sinus; vd, sperm duct; idr, veins from gill to heart. (From Ilertwig ) 



over each other so that when the animal flaps its tail 

 a sharp bend takes place and the water is pushed ahead 

 of it. At the same time the appendages move toward 

 the head so that resistance is lessened and the animal 

 moves rapidly backward. It spends a large part of its 

 time on the rocks and moves over them very easily by 

 means of the walking legs. 



