780 l'ROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, 



I am inclined to suspect the existence of a rudimentary anterior 

 arthrobranchia in VII. ; but I could not make sure of it. 



By the structure of its branchiae Stenopus is sharply separated 

 from Penceus, with which it has hitherto been associated, although 

 it approaches Penceus in the almost complete abortion of the bran- 

 chial element of the podobranchiae. In the Penaeidae, in fact, each 

 branchia consists of a stem which is pointed at both ends and gives 

 off two series of opposite lateral branches. This stem is attached 

 by a pedicle near its lower end. Near each end of the stem the 

 lateral branches are very short, and stand straight out ; but they 

 rapidly become longer ; and as they do so they curve outwards to- 

 wards one another, and eventually meet in the middle line. The 

 middle of the branchia consequently assumes the form of a hollow 

 cylinder. 



The outer face of each lateral branch gives off a close-set series 

 of secondary branches, which diminish in size towards the free end 

 of the lateral branch, and at the free end are simple undivided^ 

 filaments. But towards the attached end of the branch the secon- 

 dary branches are themselves dichotomously subdivided in the direc- 

 tion of their length ; so that the most complicated of these secondary 

 branches presents a short stem whence two branches proceed, each 

 of these again gives off two, and these may terminate in yet other 

 two. Hence each secondary branch is like a flat triangular plate 

 slit by fissures of varying depths, and attached by its apex to a 

 lateral branch. All these secondary branches are directed upwards 

 and outwards. 



A detached lateral branch closely resembles one of the branchiae 

 of Thysanopoda ; while, so far as I can judge from the figures given 

 by Kroyer 1 and Sars 2 , the branchiae of Sergestes and Lophogaster 

 still more closely approach those of Penceus. 



A similar structure was described by Duvernoy in Aristceus, and 

 wp.s supposed by him to be characteristic of that genus ; but Dana 

 has already justly expressed a doubt whether, in this respect, Aris- 

 tceus differs from the other Penseidae. 



In Penceus, the last and the penultimate thoracic limbs present no 

 trace of podobranchiae ; and in all the rest the podobranchia is reduced 

 to a small epipodite, which, in the middle of the series, is bifurcated 

 at its free end. The latter lies between the arthrobranchiae of its own 

 somite and the next? following. There are seven pleurobranchiae, of 

 which the hindermost is the largest, while the most anterior is very 

 small. 



1 Forsog til en monographisk Fremstilling af Kraebsdyrslagten Sergestes : 

 1855. 



2 Beskrivelse over Lophogaster fi/picus : 1862. 



