416 



TEXT-BOOK OF ZOOLOGY 



Ft. 



Besides, the animal is compelled to hold fast its prey, which otherwise 

 would soon be washed away, especially in running water. 



2. Accordingly, the 

 crayfish must be pro- 

 vided with organs for 

 holding and dividing 

 its prey. Such organs 

 are the large pincers 

 or claws of the first pair 

 of thoracic legs (see 

 Section vi.), which, on 

 account of their great 

 strength, serve also as 

 prehensile and defen- 

 sive organs. They are 

 formed on the general 

 plan of shears, viz., by 

 the last joint of the 

 limb being movably 

 opposed against a pro- 

 longation of the penul- 

 timate joint. These 



Jffo, pincers cannot,, how- 

 ever, be brought in 

 contact with the mouth 

 (show this by trial), 

 which is effected by 



3. Themuch smaller 

 pincers of the two suc- 

 ceeding pairs of limbs, 

 by means of which the 

 detached portions of 

 food are conveyed to 



4. The mouth parts. 

 Of these there are six 

 pairs. The foremost, 

 called the mandibles, 

 consists of 'a hard- 



Vft. 



2f.2. 



TEN FIRST LIMBS OF CRAYFISH, RIGHT SIDE (SEEN FROM 

 BELOW). (Slightly enlarged.) 



F.I. and F.2., Antennules and antennae; Ok., mandibles 

 Uk.l. and Uk.2., first and second maxillse ; Kf.l., Kf.2. 



and Kf. 3., first, second, and third maxillipedes ; Bf.l. and , ,-, - ,. 



Bf. 2. , first and second ambulatory legs ; K. , branchise. 1 1 n 6 d, masticator 



portion, which is seen 



at a glance to be the principal instrument for dividing up the food. 

 (Compare with mandibles of insects possessed of masticatory mouth. 



