Fig. 246. Two stages in the development of Limulus polyphemus.
(After Dohrn.)
A. An advanced embryo enveloped in the distended inner membrane shortly before hatching; from the ventral side.
B. A later embryo at the Trilobite stage, from the dorsal side.
I., VII., VIII. First, seventh, and eighth appendages. cs. caudal spine; se. simple eye; ce. compound eye.
Shortly after this stage the embryo is hatched, and at about the time of hatching acquires a form ([fig. 246] B) in which it bears, as pointed out by Dohrn and Packard, the most striking resemblance to a Trilobite.
Viewed from the dorsal surface ([fig. 246] B) it is divided into two distinct regions, the cephalothoracic in front and the abdominal behind. The cephalothoracic has become much flatter and wider, has lost all trace of its previous segmentation, and has become distinctly trilobed. The central lobe forms a well-marked keel, and at the line of insertion of the rim-like edge of the lateral lobes are placed the two pairs of eyes (se and ce). The abdominal region is also distinctly trilobed and divided into nine segments; the last, which is merely formed of a median process, being the rudiment of the caudal spine. The edges of the second to the seventh are armed with a spine. The changes in the appendages are not very considerable. The anterior pair nearly meet in the middle line in front or the mouth; and the latter structure is completely covered by an upper lip. Each abdominal appendage of the second pair is provided with four gill lamellæ, attached close to its base.
Three weeks after hatching an ecdysis takes place, and the larva passes from a trilobite into a limuloid form. The segmentation of the abdomen has become much less obvious, and this part of the embryo closely resembles its permanent form. The caudal spine is longer, but is still relatively short. A fourth pair of abdominal appendages is established, and the first pair have partially coalesced, while the second and third pairs have become jointed, their outer ramus containing four and their inner three joints. Additional gill lamellæ attached to the two basal joints of the second and third abdominal appendages have appeared.
The further changes are not of great importance. They are effected in a series of successive moults. The young larvæ swim actively at the surface.
Our, in many respects, imperfect knowledge of the development of Limulus is not sufficient to shew whether it is more closely related to the Crustacea or to the Arachnida, or is an independent phylum.
The somewhat Crustacean character of biramous abdominal feet, etc. is not to be denied, but at the same time the characters of the embryo appear to me to be decidedly more arachnidan than crustacean. The embryo, when the appendages are first formed, has a decidedly arachnidan facies. It will be remembered that when the limbs are first formed they are all post-oral. They resemble in this respect the limbs of the Arachnida, and it seems to be probable that the anterior pair is equivalent to the cheliceræ of Arachnida, which, as shewn in a previous section, are really post-oral appendages in no way homologous with antennæ[212].
The six thoracic appendages may thus be compared with the six Arachnidan appendages; which they resemble in their relation to the mouth, their basal cutting blades, etc.