REPORT ON THE CRUSTACEA MACRURA. xxxix 



tion beinc due to the coalescence of the meros and ischium into one. and the absence of 

 the dactylos. 



In Glyphocrangon (PI. XCII. fig. i), Spirontocaris (PI. CVII. fig. i), and Nauticaris 

 (PL CVIII. fig. i) there appear to he only four joints, a fact that is clue to the fusion of 

 the basis with the ischium and meros. 



The coxa of this appendage I believe invariably supports a small and rudimentary 

 mastigobranchia, but it only in certain forms carries a podobranchial plume which is 

 attached to this pair of gnathopoda in all the Trichobranchiata, excepting the family 

 Eryonidse. 



In the Dendrobranchiata it is absent in Penseus and its nearer conveners, but it is 

 present in the deep-sea forms, such as Aristeus, Benthesicymus, and its near allies ; and 

 it is rudimentary in Haliporus. 



In the Phyllobranchiata there never is a branchial plume attached to this pair of 

 appendages, excepting in the fresh-water genus Atya. 



This same appendage has an ecphysis almost universally attached to the basis, although 

 in some instances, as in Spongicola, it is reduced to a rudimentary condition (PI. XXVIII. 

 fig. i), and sometimes it is wanting altogether as in Nauticaris marionis (PI. CVIII. 

 fig. i). 



In this division the apical termination is generally truncate and armed with spines ; 

 in some genera, as in Pontonia, Acanthephyra, Palsemon, and Paralpheus, it tapers to a 

 point ; in Notostomus it is obliquely truncate and pointed, while in Nematocarcinus it is 

 spatuliform. 



The First Pereiopoda. — The first pair of pereiopoda varies very considerably in form, 

 power, and function. In all the genera of the Trichobranchiata it is the largest and 

 most powerful of the pereiopoda, and excepting in the Synaxidea, and their parallel 

 representatives the Haplopodea among the Phyllobranchiata, it is always chelate, and 

 in these tribes it is frequently subchelate. It is often of too great a length to be of 

 use as an organ for carrying food to the mouth ; I believe it is generally only capable of 

 being used for the purpose of holding food while the smaller hands are tearing it off, and 

 carrying it to the mouth. In this way the Prawns and Lobsters feed. I also believe 

 that the great weight adds to the power of the first pair of pereiopoda, and is of further 

 value in assisting to retain or steady the animal, when, by its seizing some fixed body, 

 it is thus prevented from being easily floated away. 



It is, I think, by observing the habits of animals in ordinary conditions, that we are 

 enabled to appreciate the value of extraordinary forms in exceptional circumstances. 

 Thus in Thaumastocheles zaleuca the first pair of pereiopoda, with its long comb-like 

 fingers, is perfectly useless for conveying food to the mouth, and can only aid the 

 animal in the way that the Prawn uses its long second pair of feet, and the Soldier Crab 



