DEVELOPMENT OF LIMULUS POLYPHEMUS. 165 



Two days after (June 24th), the same egg appeared as in fig. 20. This stage may be 

 known by the differentiation of the abdomen, and the appearance of a third pair of abdomi- 

 nal limbs, together with the origin of the claws. The compound eyes also appear as two 

 white dots on the third segment of the cephalothorax. The ocelli appear still later and are 

 situated on the mandibulary, or first segment. It is thus proved that the eyes are not in 

 Limulus developed on distinct segments. The nine abdominal arthromeres, or rings, have 

 appeared, no more being added during after life; there are six well marked cephalothoracic 

 segments. The whole body of the embryo is spherical, being as thick through as it is 

 broad. The tergal side of the abdomen is about one third thicker than the pleuro-ventral, 

 the surface of the latter being very convex. The segments of the abdomen gradually in- 

 crease in size, the ninth, or rudiment of the spine, being scarcely longer than broad, and 

 those next to it being very much curved, so that towards the outer margin they are almost 

 parallel with the median line of the body. The inner claw is developed as a tubercle on 

 the inner side and at a considerable distance from the tip of the limb, in after life gradually 

 becoming as large as its fellow. The rudiments of the flagrum or outer division of the 

 fifth pair of maxillipeds, are also prominent, beginning as tubercles. At this stage the 

 chorion bursts open, the protoderm increases greatly in size, and a corresponding amount 

 of sea water is absorbed, in which the embryo freely floats. The two halves of the chorion 

 are held together by a slight hinge, and some separate from the egg. The dorsal vessel is 

 now distinct, appearing as a pale streak extending from the front edge of the cephalo- 

 thorax to near the end of the abdomen, and situated in the median depression of the body. 



In another egg (fig. 19, 19 a) observed July 4th, but which was not quite so advanced as 

 the stage just described, the chorion not having yet split apart, the six cephalothoracic and 

 nine abdominal segments are well seen, the sutures forming deep depressions in the yolk, 

 which is pale green as before, and lobulated just within the integument. The bases of the 

 limbs are nearer together, and the line between the cephalothorax and abdomen may be 

 distinctly seen, the posterior edge of the former projecting out sensibly beyond the sides of 

 the abdomen. 



June 27th, in the egg under observation June 22d-24th, the two basal pairs of abdominal 

 feet have become more prominent, being thinner and more lamelliform, the tln'rd pair being 

 still minute. 



On the succeeding day, June 28th, the curved hamulate form of the flagrum (fig. 21.y) 

 is apparent, and the lower edge of the cephalothorax is thickened and incurved. The ru- 

 dimentary lieart is more distinct than before. At this stage the embryo moults its skin. 

 It is gradually slied, the specimen under observation requiring several days for the opera- 

 tion, a period considerably longer than in a state of freedom probably, when the water is 

 better aerated ; two days after it was first observed, the skin had been drawn off the cepha- 

 lothorax and remained attached to the abdomen. It was so thin that it was simply pulled off 

 from before backwards, not exuviated as in the larva. It begins to be shed, as other specimens 

 showed me, at a little earlier period, before the claws begin to be developed, as represented 

 in fig. 18 a, z indicating the skin partially peeled off. It may then be seen floating about 

 in the protodermic fluid, entangled in the limbs of the embryo. This skin cannot be com- 

 pared with the "larval skin" of the Isopods, as I have observed it in a species of Ligia at 

 Key \yest, and in pJcera coinosa of the shores of New England, and as described by Fitz 



MEMOIRS BOST. 800. NAT. HIST. VOL. It. 42 



