220 BRITISH FOSSIL CRUSTACEA. 



" The ocelli are situated, in an earlier period of this stage than that represented by 

 fig. 6, on the underside of the head just in advance of the mandibles, and are at this time 

 white and irregular in shape, being a little longer than broad. A little later they appear 

 on each angle of the front edge of the cephalon, the edge being afterwards expanded and 

 extended, so that, as in fig. 6, they are placed a little way in from the edge. The eyes 

 also are at first rounded white tubercles, becoming in more advanced specimens black 

 along the front edge. The protoderm measures at this period *13 of an inch in diameter. 

 Of the eggs laid in the middle of May one was hatched June 30th, the majority came 

 out of the egg about the 14th July, and the remainder kept hatching until the last 

 of September in my jar. In the succeeding summer (July, 1871) the hatching jar, in 

 which the water had remained unchanged since the previous October, and with no weeds to 

 oxygenate the water, contained several embryos in eggs laid during the preceding year, 

 in different though advanced stages of development, and also several larvae hatched 

 during 1870, though in a torpid state, due probably to the little air in the water. I 

 much doubt whether the eggs of any other living Crustacean would show as much 

 vitality. It should also be noticed that those eggs had not probably grown during the 

 winter, but remained as it were in a state of suspended animation from October to July, 

 a period of nine months. These facts confirm the remarkable observations of Rev. Dr. 

 Lockwood on the wonderful vitality of the eggs of Limidu8. m (See ante, p. 213.) 



There is a considerable difference (probably due to age) between Dr. Dohrn's 

 ' Trilobiten-Stadium ' (see PL XXXIII, fig. 12) and that so called by Dr. Packard 

 (PI. XXXIII, fig. 6). Dr. Dohrn's figure agrees better with Dr. Packard's later stage, 

 the freshly hatched young (PI. XXXIII, figs. 10 and 11). The segmentation in the 

 cephalon of fig. 12 has entirely disappeared, although the hinder body is still distinctly 

 segmented, and the segments were no doubt to some extent movable. In fig. 10 (copied 

 from Dr. Packard) the division of the hinder body into segments is obliterated save along 

 the central axis. 



In its earlier stages the young Limulus can fold its posterior segments under its head- 

 shield, and when it at last quits the egg, before having undergone the first moult 

 subsequent to hatching, it is very lively, walking over the jar, and burrowing in the 

 sand at the bottom and occasionally swimming to the top, or skimming obliquely over the 

 bottom by vigorous strokes of its thoracic feet. Mr. Alexander Agassiz once captured 

 a specimen swimming on the surface three miles from shore. This fact indicates that 

 this species owes its wide range, from Maine to the Gulf of Mexico and the West Indies, 

 to the swimming habits of the larvae. 



The principal differences from the embryo just before leaving the egg are these : — the 

 thoracico-abdominal shield is larger than the cephalic one ; the sinuate sutures defining 

 the segments of the post-cephalic somites are nearly obsolete, and the hepatic lobes 

 Avhich gave so strongly-marked a character to the head-shield in figs. 4, 6, and 7, 



1 A. S. Packard, junr., op. cit., p. 168. 



