296 



Sir William Burnett, F.R.S., Director- General of the Medical De- 

 partment of the Navy. Received March 3, 1853. 



The little crustacean which is the subject of this paper was taken 

 in considerable numbers in the voyage from St. Vincent to Rio Ja- 

 neiro. There are several anatomical peculiarities mentioned, but 

 the most remarkable is the structure of the right antenna of the 

 male. These organs are in the female perfectly symmetrical, and 

 resemble that of the left side in the male ; and although in the very 

 young state of the latter sex the right antenna differs but little in 

 external appearance from the left, yet the peculiar hypertrophied 

 condition of the modified segments in the corresponding organ of 

 the adult male is to be distinctly traced in a rudimentary state. 



As the animal lives in the open ocean, none of the limbs are 

 adapted for walking ; but when placed in a vessel of sea- water, they 

 rested upon their antennae on reaching the bottom, and paddled 

 themselves about by their fore-limbs and tail. 



The author remarks that in all their movements the males exhibit 

 a tendency to turn towards the left side, and concludes the rationale 

 of this fact to be, that the brain on the right side being more deve- 

 loped at the part from which the right antenna derives its nerves, a 

 corresponding predominance is given to the power of the locomotive 

 organs on that side. 



When fully developed, each antenna in both sexes consists of 

 twenty-five segments. Of these, the first thirteen present nothing 

 remarkable ; but all the remaining pieces on the right side enter 

 into the composition of the curious prehensile organ which forms 

 the principal subject of the paper. 



This organ is composed in the following manner: — The fourteenth 

 and four following segments are dilated into a large flask-like organ, 

 the neck of which is eked out by the nineteenth and twentieth. 

 The next two segments are fused together, and are articulated with 

 the foregoing by a simple joint, and the whole of the remaining 

 segments form another piece similarly articulated with the inter- 

 mediate piece ; so that the whole results in two simple joints sus- 

 ceptible of flexion in one direction only. On the eighteenth segment 

 is a barbed process having its apex directed backwards, and its an- 

 terior border beset wdth sharp teeth. Two processes of the same 

 nature, but differently placed and more elongated, lie side by side 

 upon the fore-part of the first compound segment. This piece and 

 that which succeeds it act upon each other like a pair of jaws, each 

 furnished with an array of sharp conical teeth, w^hile the last com- 

 pound member of the series plays over the upper surface of the 

 eighteenth segment. 



The author then proceeds to describe the muscles which move 

 this complex apparatus. The extensors are small and feeble, but 

 the flexors are, as might be anticipated, more complex and power- 

 ful. They are two in number. The first has its origin in the large 

 flask-like dilatation, and is inserted by a tendon into the second 

 compound piece, from which the second muscle arises, and is in- 

 serted, also by tendon, into the third piece. The paper is illustrated 

 by elaborate drawings. 



