344 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



III. METAMORPHOSIS, OR PERIOD FROM THE TIMK OF HATCHING TO THE ADULT STATE. 



A. Protozoea or first larva (length =4 mm ). Steuopus leaves the egg as a protozoea, which may 

 be compared to one of the early larva? of Peun?us or Sergestes, but it is unlike either of them. 

 Tins first larva,which is very long and slender, is so coiled upon itself in the egg that the tail fin 

 overlaps the posterior end of the carapace. It requires considerable time after casting off the 

 shell to uncoil and straighten its appendages, especially the antenna; and the long rostrum which 

 was bent under its body. 



The figure on PI. xi exhibits some of the grotesqueness of this larva. This drawing was made 

 from an animal which had just wiggled out of its egg shell and was uncoiling its appendages. 

 The huge antenna? are partially unfolded, while the rostrum R., is scarcely visible. Drawings of 

 parts of this immature protozoea are seen in PI. vti, Figs. 11-10, and the larva itself as it finally 

 appears, about two hours after hatching, in Fig. 11. If we compare with this the younger form in 

 Fig. 25, we notice some details, chiefly of a quantitative kind, in which .they differ. 1 in mediately 

 after leaving the egg the epidermic structures grow rapidly ; hairs or seta? are developed on all the 

 appendages, and the tail-flu acquires some new characters. The first larva does not swim well 

 until several hours after hatching. 



The Steuopus protozoea (PI. vn, Fig. 11) is 4 mm long, the rostrum alone being 1 J mm . It is color- 

 less, excepting the dark eyes and a few scattered blotches of brownish pigment upon the sides of the 

 body or on the tail-fin. It swims chiefly by aid of its largely developed antenna?, which are directed 

 forward as shown in the plate. These, with the rostrum, add considerably to the apparent length 

 of the body and serve to distinguish it, without the aid of a lens, from the second larva (PI. vin, 

 Fig. 17), which soon follows and swims about in the aquarium with the others. It is further character- 

 ized by the very large size of its mandibles (PI. xi, Fig. 25, Md.) and by its forked telsou-plate, 

 adapted for swimming. The forked locomotor tail-fin and large hairy antennae mark the protozoea 

 stage in Crustacea generally. The carapace is only feebly developed, not nearly reaching to the bases 

 of the appendages. It is prolonged in front into a huge tapering cone, the rostrum, which is nearly 

 half the length of the body. This is beset with short spines and reaches considerably beyond the 

 antenna?. About four segments of the abdomen are distinguishable from before backwards (Fig. 25). 

 The first and second, which latter is the largest, carry lateral spines, and the upper surface of the 

 second segment is also prolonged posteriorly into a median spine. The tail-fin at the time of hatch- 

 ing is sharply forked (Fig. 13) and is furnished with 6 pairs of rudimentary seta?, of which the 

 median pair is the shortest, besides a pair of outer non-plumose bristles (Figs. 11, d, and 13, a.). In 

 the course of a few hours this organ has become functional and appears as shown in Fig. 11. The 

 hairs grow out and acquire thick lateral fringes; the outer pair (next to a) become rudimentary, 

 and three additional pairs of toothlike bristles make their appearance on the sides of the telsou- 

 plate. 



The eyes are sessile. The inner or first antenna? (Fig. 25, AI) are jointed, unbranched append- 

 ages. Each is tipped with a bunch of about four long sensory filaments and with a single seta. A 

 single plumose hair also springs from the distal end of the penultimate joint on its inner side. 

 The outer antenna? are birauious. The inner branch consists of a simple stem, tipped with at 

 least two long hairs. The outer divisiou is segmented at its extremity, and is garnished with 

 plumose seta?, chiefly on the inuer margin, there being one or two to each segment. The gland at 

 the base of the auteunal peduncle is conspicuous. 



The mandibles are of enormous size in comparison with the other appendages. A view of the 

 labruni and right mandible is given in PI. vn, Fig. 15. They are simple blades with rounded 

 edges, covered with minute horny teeth. There is no palpus. The first maxilla (Fig. 12) consists 

 of two stout branches tipped with bristles, and in the case figured they are spotted with pigment. 

 The second maxilla is a broad lobulated plate (Fig. 10). Each lobe is provided with hairs, 

 excepting the outermost which corresponds, in part certainly, to the scaphognathite. Ouly a 

 single bristle was detected on This lobe in the specimeu from which the drawing was made. 



The three maxillipeds have each an exopodite, which is considerably larger than the other 

 branch, and which is furnished near the tip with not less than three pairs of locomotor hairs. The 

 undeveloped condition of these latter in au embryo just hatched but unable to swim, is well shown by 



