DEVELOPMENT OF LIMULUS POLYPHEMUS. ]69 



never had any contact with the sunlight. At once, not without some misgivings as to 

 the result, the proper provision was made to complete the incubation, namely, new sea 

 water, clean sand, the eggs put on top and all set in a favorable place. With an ordinary 

 hand lens the progress of incubation could be observed daily. At half past four o'clock on 

 the afternoon of May 11th, before my eyes, a new-born baby Limulus left the egg. Just 

 think of it — these eggs are within two weeks only of being a year old ! And then how re- 

 markable are these facts also; these eggs were incubated last summer! Hence there has 

 been not only a remarkable retardation of development, but also an actual arrest of the 

 same for seven or eight months without sacrificing life." 



The larva state. The larvse, as we shall call the young before they have undergone 

 then- first moult, are 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 their abdominal feet. Mr. Alex. Agassiz informs me that he 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 larvse. The principal differences from the embryo 

 just before leaving the egg are these — the abdomen is larger in proportion to the cephalo- 

 thorax, it being a little broader and longer ; the sinuate sutures defining the segments of 

 the abdomen are nearly obsolete ; the lobes of the liver do not reach so near the edge of 

 the carapace, and the edge is fringed with hairs ; these differences being very slight. 



The cephalothorax is about one-half as long as wide, each half being a quadrant of a 

 circle. The integument is pale horn color and very transparent, while the liver and car- 

 diac region of the abdomen are pea green ; minute granules are scattered over the integu- 

 ment. The edge of the carapace is fringed with fine, slender hairs, thirty-six on each side ; 

 they are wanting in a short space on the median line of the carapace, and at the posterior 

 angle, and on the hinder edge. They do not arise from the extreme edge, but from little 

 sockets placed on the upper side of the edge. They project upwards and forwards and hence 

 lie back on the carapace when the animal is burrowing. Just within this hair-bearing 

 edge is a solid space, or line, its inner edge being the wall of the general cavity of the 

 body, the blood circulating next to it. The edge of the carapace along its posterior half 

 is minutely toothed ; there being about five saw-like teeth between the hairs, the teeth 

 pointing backwards. The "frontal doublure" or triangular area in front of the mandibles is 

 not seen from above, but beneath is quite small compared with the same part in the adult. 

 The ophthalmic ridge (so named since the eyes and ocelli are situated upon it, and as 

 it separates the cardio-ophthalmic region from the lateral region of the carapace) is distinct, 

 but still much slighter than later in the larval stage, and without the teeth overhanging the 

 eyes (though there is a swelling instead), and also without the median tooth on the posterior 

 edge of the cephalothorax ; this appears, however, before the animal moults. The eyes are 

 oval, and placed obliquely, the posterior end being directed inwards ; the anterior three-fifths 

 of the surface is black, the pigment cells are continued in front of the eye along the ridge, 

 forming a faint black line. The ocelli are two round black dots separated by a space twice 

 their diameter in width. In a transverse section, the cardiac region is very prominent, con- 

 vex, carinated, and, with the high cardio-ophthalmic region and rapidly sloping edge, re- 

 sembles strilvingly the glabella of a trilobite. The cardiac region of the abdomen is convex 

 and carinated, and retains the sutures between the segments much more distinctly than the 



MEMOIRS BOST. SOC. NAT. HIST. VOL. II. 43 



