322 CRUSTACEA. 



annulose skeleton may be detected even in the most compactly 

 formed CRUSTACEA, it remains for us to inquire, in the next 

 place, what are the principal modifications observable in the arti- 

 culated appendages developed from the individual segments. This 

 inquiry is one of considerable interest, inasmuch as it goes to prove 

 that, however dissimilar in outward form, or even in function, the 

 limbs of Crustaceans are mere developements of the same elements, 

 which, as they remain in a rudimentary condition or assume larger 

 dimensions, become converted into instruments of sensation, legs, 

 jaws, or fins, as the circumstances of the case may render needful. 

 In the lower, or more completely annulose forms (figs. 14$ and 

 152), these members are pretty equally developed from all the seg- 

 ments of the body, and are subservient to locomotion, being gene- 

 rally terminated by prehensile hooks, or provided with fin-like ex- 

 pansions ; but, as we advance to the more perfect genera, the limbs 

 assume such various appearances, and become convertible to so many 

 distinct uses, that they are no longer to be recognised as consisting 

 of similar elements, modified only in their forms and relative pro- 

 portions. To notice Fig. 152. 

 all the varieties which 

 occur in the extensive 

 class before us, would 

 be to weary the reader 

 with tedious and unne- 

 cessary details ; we shall 

 therefore select the De- 

 capod* division of these 

 animals, as abundantly sufficient for the illustration of this part of 

 our subject. This division, which includes the most highly organ- 

 ized forms, has been divided by writers into three extensive fami- 

 lies, the Macroura, or swimming Decapods ; the Anomoura, which 

 inhabit the empty shells of Mollusca ; and the Brachyura, or short- 

 tailed species, of which the crab is a familiar specimen. If we take 

 the common lobster as an example of the first of these groups, we 

 shall find that there are five pairs of articulated limbs placed upon 

 each side of the mouth, which are evidently adapted to assist in 

 seizing and conveying into the stomach substances used as food. 

 These singular organs, which, although entitled to be considered as 

 jaws so far as their use would indicate the name belonging to them, 



* So called from the circumstance of their having five pairs of limbs so largely de- 

 veloped as to become ambulatory or prehensile organs. 



