MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 363 



THE THIRD LARVAL STAGE. 

 (PI. xvi. Fig. 1.) 



After molting the second time the larva assumes the form shown in PI. xvi, Fig. 1. It is also 

 shown, much less enlarged, in side view in PI. xvii, Fig. 1. The first and fifth thoracic limbs are 

 DOW functional, the second is represented by a bud, all the abdominal somites are distinct, and 

 the sixth abdominal appendage has made its appearance. The first five abdominal appendages 

 are still unrepresented, and the endopodite of the sixth is rudimentary, although its exopodite is 

 fully developed and functional. 



Those appendages which were present in stage two have undergone little change. The external 

 branch of the antenuule has, in place of the four seuse-hairs of the earlier stage, only two, which 

 are much longer than before. The long terminal hair of the inner branch has lost the marginal 

 hairs of the earlier stage aryl is now simple, while two plnino.se hairs have made their appearance 

 on the lower surface of the distal joint of the shaft. The scale of the antenna is still cylindrical, 

 but the annulations which marked it during the earlier stage have disappeared. The flagellum 

 still consists of only one short joint, and the long terminal hair which it carried at the earlier stage 

 has disappeared. The mandibles, maxillae, and maxillipeds are about as they were before, but the 

 eudopodite of the third maxilliped has almost completely lost the long terminal hair of stage 

 two, and has also become relatively shorter, and is now divided into four joints. 



The first thoracic leg, which was rudimentary in stage two, has now acquired a flat basal 

 joint and a plumose exopodite, like those of the preceding appendages, but the endopodite is 

 represented only by a rudimentary knob or bud upon the anterior edge of the basal joint. The 

 second thoracic limb is, as it was at the earlier stage, a two-lobed bud. No buds have as yet 

 appeared between it and the base of the fifth thoracic appendage, which is now fully developed 

 and forms the most conspicuous peculiarity of this stage in the development of Alpheus. It has 

 no exopodite, its basal joint is not enlarged nor flattened, and its long, slender, cylindrical shaft, 

 made up at this stage of four joints, is prolonged at its tip into a long, slender, tapering, simple 

 hair, the end of which reaches beyond the tips of the antenna? when the appendage is in the posi- 

 tion shown in the figure PI. xvi, Fig. 1. The appendage seems to have little power of motion and 

 it seldom deviates much from the position shown in the drawing, being usually carried closely 

 pressed against the ventral surface of the body between the liases of the other appendages, with 

 its tip directed forward. All six abdominal somites are distinct and movable, but the first five 

 have as yet no traces of appendages. The first four somites are short and equal, the fifth is nearly 

 as long as the first four together, and the sixth is very narrow and almost twice as long as the 

 fifth. The eudopodite of the sixth abdominal appendage is present and of considerable size, but 

 it is not as yet functional, although the exopodite, which is not very much larger, is fringed by 

 six long, plumose, swimming hairs and is used in locomotion. The two spines which are carried 

 upon the lateral margins of the telsou at an earlier stage have disappeared, and there is less dif- 

 ference than before in the relative sizes of the others, but the general form is the same. 



FOURTH LARVAL STAGE. 



The subsequent history of Alpheus minor was traced by one of the authors at Beaufort and 

 by the other at Nassau, but as the stages which follow were found to be almost exactly like the 

 corresponding stage of other species which had already been drawn, it did not seem to be advisa- 

 ble to make new figures, and in the remainder of the description the illustrations which are 

 referred to actually represent the larva; of other species. After its third molt the larva of 

 Alpheus minor passes into its fourth stage, when it becomes almost exactly like the fourth larval 

 stage of Alpheus heternchelis, shown in PI. xvm, Fig. 3. There is little change at the anterior end 

 of the body, except that the carapace now begins to extend over the eyes, and the ears have made 

 their appearance in the basal joints of the antennnles. The mandible has lost its outer branch, 

 and the basal joint of the second maxilla, PI. xvi, Fig. 5, carries on its inner edge three hairy lobes. 

 There are now five pairs of swimming appendages in place of the three of stages one and two, and 

 the four of stage three. These five are the exopodites of the first, second, and thud maxillipeds 



