170 A. S. PACKARD, JR., ON THE 



sides, and the green yolk mass beneath, not entirely absorbed, is also deeply subdivided and 

 lobate on the edges. The edge of each segment is toothed, the basal tooth being the 

 largest, the succeeding serrations being fine, and with a moveable spine inserted on the 

 hind edge of the tooth, and projecting beyond it. Between these teeth are three or four 

 (the number varies) very minute short hau-s. It is .13 of an inch in length and .11 of an 

 inch in breadth. 



The legs are much as in the adult, the coxal spines, with the exception of one or two, 

 not being developed until after several moults. The fifth maxilliped (fig. 23, c) has six 

 spines on the penultimate joint, which have not yet assumed their spatulate form, but the 

 terminal forceps is much as in adult life. The first pair of gill-bearing abdominal legs 

 (fig. 26, a) are now much longer than before, with a long, dense fringe of fine hairs. Each 

 leg is semi-ovate in form, with no appendages, and with a suture at the basal tliird extend- 

 ing two-thirds of the distance from the outer edge to the inner, where it forks, enclosing a 

 triangular area, covered with a few hairs, increasing in length towards the inner edge of 

 the leg where it is coalesced with its fellow, having been separated in embryonic life. The 

 two legs are united to form one piece, dividing on the outer third. The second pair reach 

 a little beyond the edge of the basal pair, and resemble those of the embryo, beai'ing a dis- 

 tmct jointed limb. Fig. 25, h, represents the lai'va in a front view ; 25, c, in a back view, 

 with the abdomen bent with the cephalothorax ; and 25, d, a profile of the body when ex- 

 tended, for the sake of comparing it with similar positions of the Trilobites. 



The young after the first moult. The larva moults its skin during the last week in July, 

 about the 25th of the month, or about three weeks after hatching. The principal changes 

 are the possession of a short abdominal spine, the origin of a fourth pair of abdominal 

 legs and the appearance of additional joints to the basal ones, and the smaller, but more 

 numerous lobes of the liver, which do not reach so near the edge of the carapace as in the 

 larva, or as in the adult when a foot in length. 



The eyes are as in the larva, partially black. The abdomen now more nearly resembles 

 that of the adult ; the acute side of the basal segment forms a revolute spine, and the suc- 

 ceeding teeth are much larger than in the larva. The spines are minute and slender and 

 scarcely extend beyond the teeth, though so much larger in proportion in the adult. The 

 terminal spine is inserted in a deep lobe, or sinus ; in the eighth ring the lateral spine on 

 each side reaching to its middle. It is broadly ensiform, being subacute and about three 

 times as long as broad ; it is thin, hollowed out beneath, the convexity being angular. The 

 base of the spine is situated at quite a distance from the edge of the abdomen, embracing 

 the anus which opens in it. 



The fourth pair of abdominal legs are small, and concealed under the basal ones. The 

 first pair differ from those in the larva in being longer in proportion to their width, and in 

 having an additional joint at the base, the suture extending about half way in to the inner 

 edge. The two limbs are united more closely than before, dividing much nearer their ex- 

 tremities. The second pair are now three-jointed, each leg with a slender, two-jointed, free 

 tarsal termination, and a lobe just external to it. There are five lamellge to the gills, at- 

 tached to the basal joint. The fringe of hairs is much shorter than on the basal pair. 



It is .25 of an inch in length. The circulation of the blood (PI. v, fig. 27) can be 

 observed much better after the young Limulus has moulted its integument once, the body 

 being more transparent than before. The heart, already described by Van der Hoeven, 



