In the course of the next few days the two first appendages of the abdominal region become formed (vide [fig. 245] C shewing those abdominal appendages at a later stage), and have a very different shape and direction to those of the cephalothorax. The appendages of the latter become flexed in the middle in such a way that their ends become directed towards the median line ([fig. 245] B). The body of the embryo ([fig. 245] B) is now distinctly divided into two regions—the cephalothoracic in front, and the abdominal behind, both divided into segments.
Fig. 245. Three stages in the development of Limulus polyphemus.
(Somewhat modified from Packard.)
A. Embryo in which the thoracic limbs and mouth have become developed on the ventral plate. The outer line represents what Packard believes to be the amnion.
B. Later embryo from the ventral surface.
C. Later embryo, just before the splitting of the chorion from the side. The full number of segments of the abdomen, and three abdominal appendages, have become established; m. mouth; I-IX. appendages.
Round the edge of the ventral plate there is a distinct ridge—the rudiment of the cephalothoracic shield.
With the further growth of the embryo the chorion becomes split and cast off, the embryo being left enclosed within the inner membrane. The embryo has a decided ventral flexure, and the abdominal region grows greatly and forms a kind of cap at the hinder end, while its vaulted dorsal side becomes divided into segments ([fig. 245] C). Of these there are according to Dohrn seven, but according to Packard nine, of which the last forms the rudiment of the caudal spine.
In the thoracic region the nervous system is by this stage formed as a ganglionated cord (Dohrn), with no resemblance to the peculiar œsophageal ring of the adult. The mouth is stated by Dohrn to lie between the second pair of limbs, so that, if the descriptions we have are correct, it must have by this stage changed its position with reference to the appendages. Between the thorax and abdomen two papillæ have arisen which form the so-called lower lip of the adult, but from their position and late development they can hardly be regarded as segmental appendages. In the course of further changes all the parts become more distinct, while the membrane in which the larva is placed becomes enormously distended ([fig. 246] A). The rudiments of the compound eyes are formed on the third (Packard) or fourth (Dohrn) segment of the cephalothorax, and the simple eyes near the median line in front. The rudiments of the inner process of the chelæ of the cephalothoracic appendages arise as buds. The abdominal appendages become more plate-like, and the rudiments of a third pair appear behind the two already present. The heart appears on the dorsal surface.
An ecdysis now takes place, and in the stage following the limbs have approached far more closely to their adult state ([fig. 246] A). The cephalothoracic appendages become fully jointed; the two anterior abdominal appendages (VII.) have approached, and begin to resemble the operculum of the adult, and on the second pair is formed a small inner ramus. The segmentation of the now vaulted cephalothorax becomes less obvious, though still indicated by the arrangement of the yolk masses which form the future hepatic diverticula.