DESCRIPTIONS OF GENERA AND SPECIES. 



Suborder MACRURA. 



This; section of the order Decapoda in the Crustacea may be generally denned by the 

 following: external characters : — 



The animal is elongated and compressed. The carapace is less than half the length 

 of the animal, and is anteriorly produced in the median line and covers the ophthalmic 

 somite. 



The ophthalmopoda are not enclosed within orbits, and rest in a hollow in the 

 upper surface of the first joint of the peduncle of the antennae. 



The first pair of antennae is elongated, and not planted in fossettes. 



The second pair is considerably elongated, and carries a foliaceous appendage attached 

 to the second joint. 



The second pair of gnathopoda is elongated and pediform. 



The pereiopoda have seven distinct joints, of which the coxa articulates with the 

 pereion. 



The pleopoda consist of biramose appendages, of which the anterior pan varies from 

 the succeeding, and the posterior is associated with the telson and helps to form the great 

 caudal fan or rhipidura, which is the only feature that is invariably constant and 

 common to all families of the suborder. 



Variations both in the structure and in the relative importance of parts occur in most 

 organs, in some to a considerable extent, but the passage from one modification 

 to another clearly demonstrates that such changes are of specific or generic value only. 

 This is well shown in some species of Pentacheles, where some have the branchial lash 

 (mastigobranchia) large, others small, and in some it is wanting altogether, and this 

 variation occurs in specimens which differ little in external appearance, and which were 

 procured in some instances from the same locality. 



One of the most conspicuous variations of structure, and most convenient for 

 examination, is to be found in the branchial appendages. In some genera these organs 

 are developed as a series of leaf-like plates, in others they exist as feather-like plumes of 

 slender cylinders, and again they are found to divide into a series of tree-like branches, 



(ZOOT* CHALL. EXP. PART LII. — 1886.) 1" *• 1 



