82 INTRODUCTION TO ZOOLOGY. 



the bases of which perform the office of jaws; and for its 

 reception a third division — Xiphosura* has been specially 

 constituted. 



Reverting to our native species, we find some, as already 

 mentioned (page 76), with the eyes on footstalks, others with 

 the eyes sessile. This forms an excellent characteristic dis- 

 tinction. Again, some have the gills enclosed in the body, and 

 have tenlegs ; others have the gills external, and the number of 

 the legs or appendages variable. By such characters they are 

 divided into sections, orders, sub-orders, genera, and species. 

 All of those which are the best known and the most valued, 

 are, with regard to their food, masticating [Maxillosa) ; have 

 the eyes on footstalks {Podoptkalma)', and have ten legs 

 {Decapoda).-\ These scientific terms, though startling to 

 beginners, do nothing more than express, in a difl'erent form, 

 the same meaning that the simple English words convey. 



The animals composing the first group we shall mention 

 among our native Crustacea, familiarly known as *' Spider- 

 crabs, " from their length of legs. Mr. W. Thompson gives an 

 instance of one of them {Hf/as aranea) only two and a quarter 

 inches across the "shell" which had an oyster three inches in 

 diameter upon his back, and remarks that the Crab must have 

 enacted the part of Atlas for some successive years, as the oyster 

 was encrusted with large acorn-shells, and could not have been 

 less than five years old. | A series of such observations would 



• Sword-tailed. Figure 56 represents the lower surface of the animal, 

 m, the Mouth — -f, Feet, the basis of which perform the office of jaws. — 

 a, Abdominal appendages bearing the branchi£e.— ^, Sword-shaped tail. 



t In the ten-footed Crustacea (Decapoda), there is a striking dif- 

 ference in the form and development of the tail, as in the Crab and in 

 the Lobster; and they are thus divided into two very natural groups. 

 The Hermit-crabs, in which the tail is prolonged, but defenceless, may be 

 regarded as a connecting link. Hence, Milne Edwards, in his excellent 

 " Histoire des Crustaces," arranges them in three sections, distinguished 

 by terms expressive of these peculiarities of structure. Thus: — 



DECAPODA. 



1st section, Brachyura, or short-tailed, as the Crabs. 



2d „ Anohioura, or irregular-tailed, as Hermit-crabs. 



3d „ Macroura, or long- tailed, as the Lobster, Cray-fish, &c. 



X The information given in this page, and acknowledged elsewhere, 

 by the initials, W. T. is derived almost exclusively from a paper on 

 "the Crustacea of Ireland, order Decapoda," by AVilliam Tliompson, 

 Esq.; President Nat. Hist. Society, Belfast, published in Annals Nat. 

 Hist. vols. X. xi. 1S42-3 ; and we have not scrupled, on many occasions, 

 to avail ourselves of the language there employed. 



