132 



The Naturalist. 



coarsely toothed spine directed backwards, the length of which varies 

 with the age of the individual, being longest at birth. The entire 

 surface of this bivalved carapace is ornamented with rectangular 

 markings, in the centre of each of which an opaque speck is often 

 present, especially in adult specimens, giving to the shelly covering a 

 shagreened appearance. These diamond markings are of specific 

 value, as by them the D. pulex may be distinguished from all other 

 members of the genus. 



The large inferior antenna3 are quite independent of the carapace, 

 springing directly from below the point where the cephalic and bivalve 

 portions of the carapace unite. Each antenna arises- by a thick base 

 having a ball-and-socket-joint : it subdivides into two jointed branches, 

 each half the diameter of the base, and consisting of three articula- 

 tions of about equal length. From the sides of the first and second 

 joints respectively in one, and the second only in the other, arises a 

 long slender filament, and from the tip of each branch three similar 

 filaments, all of equal length, jointed about their middle, and 

 delicately plumose along their margins. These large arborescent 

 antennae are the chief if not the only organs of locomotion. Close 

 behind and parallel with the rostrum is located a pair of small 

 antennae, unjointed but tipped with ten short filaments. Within these 

 superior antennse are lodged the olfactory organs. 



The five pairs of jointed feet are partly branchial and partly 

 alimentary in function. Although furnished with numerous articula- 

 tions, and highly plumose, ihey are adapted for neither crawling, 

 swimming, nor indeed, are they capable of assisting the movements of 

 the animal in the least. Whilst the individual is at rest, the feet are 

 ' incessantly in motion, communicating to the surrounding fluid an 

 undulatorv flow which bathes their surfaces and aerates the blood 

 circulating in large oval branchial plates situated behind the third, 

 fourth, and fifth pairs. 



The myology of this little creature can be readily followed through 

 the transparent integument. The limbs, as in all Arihropoda, are 

 hollow, and the muscular fibres to the several joints can be distinctly 

 traced. The principal muscles are faintly striated transversely, and 

 their arrangement is, for the most part, exceedingly simple. Those of 

 the arms, mandibles, thoracic limbs and tail all arise from the first 

 articulation immediately in front of and below the heart, whence a 

 broad striped muscle proceeds to the dorsal margin of the carapace 

 above and in front of the heart. The muscles of the abdominal limbs 



