476 



ZOOLOGY. 



division of the food (g, 143, 144) ; those of the middle por- 

 tion of the body constitute limbs for locomotion, and those 

 placed still further back have very variable uses, but serve in 

 general for respiration or reproduction : finally, this long 

 series terminates generally by one or more pairs of limbs, 

 arranged to serve as fins. 



The head, or rather the cephalic 

 portion of the body, carries the 

 eyes, the antennae, and the buccal 

 appendages < sometimes it is di- 

 vided into several distinct rings, 

 as in the squilli or shrimps, for 

 example (Fig. 515) : although 

 generally it presents no such sepa- 

 ration, being formed only of a 

 single segment, which seems to 

 represent seven rings, confounded 

 together. Sometimes it is move- 

 able, and distinct from the thorax 

 (Fig. 505) ; at other times, on the 

 contrary, it is united to this second 

 portion of the body, which in its 

 turn is composed in certain species 

 of rings articulated together, but 

 distinct ; in others, united into a 

 single mass. 



The antennae are almost always 

 composed of two pairs, and con- 

 stitute in general very elongated filiform horns, or what 

 at least resembles them. The limbs are connected in pairs 

 with the different thoracic rings; there are frequently 

 seven pairs: in the cloportes (Fig; 505), the prawns of 

 rivulets, and the talitri, or sand-hoppers, for example ; but 

 at other times, as may be seen in the crabs (Fig. 506) and 

 the craw-fish and lobsters (Fig. 143), their number is re- 

 duced to five pairs only ; for the appendages, which in the 

 first case formed the four anterior limbs, are then turned to 

 other uses, and transformed into organs of mastication. There 

 exist also very great differences in their structure ; in some 

 Crustacea they are wholly foliaceous, membranous, and exclu- 

 sively adapted for swimming- (Fig. 518) ; in others they have 

 the form of small flexed (like elbows) columns, articulated, 

 and disposed only for walking ; in others still, besides re- 



Fig. 508. Hippa. 



