CRUSTACEA. 



759 



.Fig. 388. 



o, second thoracic ring of the Squilla ; b, one of 

 the small antennas. 



pendages belong especially to the function of 

 sensation, and as we shall have to revert to 

 these at a later period, and give an ample de- 

 scription of their structure, we shall not enter 

 upon this subject farther at present. 



Fig. 389 



Third and fourth cephalic rings of the Squilla : 

 a, carapace ; b, one of the posterior antennae j 

 c, one of the mandibles. 



The eleven pairs of appendages which suc- 

 ceed are variously apportioned between the 

 functions of digestion and locomotion, to which 

 last the five hindmost pairs are entirely dedi- 

 cated in the Decapods. In other Crustacea, 

 again, the first pair only is set apart in an 

 especial manner for the office of mastication, 

 all the others then serving for locomotion, and 

 this pair is in consequence very generally de- 

 scribed under the name of mandibles ; very 

 commonly one and even two other pairs are 



added to this first pair, and these are desig- 

 nated jaws or maxilla. In the majority of 

 instances, moreover, the three succeeding pairs 

 assist the three preceding; and as they are 

 frequently more especially apportioned to loco- 

 motion, the two last in particular, whilst in 

 some cases -they serve for the two functions at 

 one and the same time, they have been de- 

 signated by anatomists and naturalists the 

 maxillary limbs (pieds-machoirs): these we 

 shall describe when we come to speak of the 

 apparatus of digestion. 



As to the five pairs which we have already 

 mentioned as essentially ambulatory (see 

 jig. 382), they present in the Brachyura no 

 more than a simple stem, composed of six 

 articulations ; whilst in the Astacus and allied 

 genera, we find a flabelliform appendage or 

 whip, dedicated especially to the purposes of 

 respiration, and in the Peneae the three sorts 

 of appendages existing simultaneously. By- 

 and-by, when speaking of respiration, we shall 

 see how it happens that in a great number of 

 these animals the whip of the thoracic extremities 

 assumes a vesicular structure, and becomes 

 the organ of this important function. 



The same peculiarity is observed in the 

 appendages of the abdominal extremities 

 of a great number of species; but among 

 the members of the most elevated tribes, 

 these appendages are but very slightly 

 developed, and appear to have no other 

 use than to attach the eggs along the in- 

 ferior surface of the abdomen. 



Fig. 390. 



Abdomen of the female Maja 



Squinado. 



a, the abdominal appen- 

 dages. 



We shall not at present enter upon the con- . 

 sideration of the forms of the thoracic and 

 abdominal extremities, having it in view to 

 take up the subject when we come to examine 

 these appendages as the organs of prehension, 

 and as fulfilling important offices in locomotion. 



Before quitting the study of the tegumen- 

 tary skeleton, to go on to that of the extre- 

 mities considered especially as the organs of 

 locomotion, we think it necessary to say a few 

 words upon the moult or process by which the 

 tegumentary covering of the whole of the Crus- 

 tacean is cast off and renewed. 



The necessity for this operation is a con- 

 sequence of the very nature of the envelope : 

 like every other epidermic covering, the pro- 

 duct of secretion, the shell of the Crustacea is 

 closed in on every side, and can only increase 

 in thickness, so that all growth would be pre- 

 vented in the body of these animals were they 

 denied the power of freeing themselves from 

 time to time of their prison. Accordingly they 

 have this power; and as might have been ex- 

 pected the shell is cast by so much the more 

 frequently as the animal is younger, inasmuch 



