DEVELOPMENT OF LIMULUS POLYPHEMUS. 167 



under this form. The embryo of these Lerneans then passes successively through the phases 

 of blastoderm, of nauplius and of cyclops, and at each of these states, it secretes at its 

 surfiice a cuticular membrane ; it undergoes successively a blastodermic moult, a nauplian 

 moult, and a cyclopean moult." 



It is probably with this cyclopean skin that the moulted skin of Limulus is homologous. 

 But I should venture to differ from M. Van Beneden in considering that any Copepod 

 with simple eyes and no distinct abdomen and head, such as Cyclops and others, sheds a 

 zoea skin, as I would limit the term zoea to the larvae of the Decapoda,_and such Branchi- 

 opoda as possess in the larval state a pair of compound eyes, in addition to simple ones, and 

 that have more than three pairs of feet, such as most nauplii have, and furthermore possess 

 an abdomen more or less differentiated from the head, or, when it exists, cephalothoi'ax. 

 Examples of such zoese among the Branchiopoda are the later larval condition of the Bran- 

 chiopodidae, Nebaliadae, and the larvoe of Limulus and the Trilobites. 



A little farther on in its development the body, from being spherical, flattens and 

 grows wider, the sides of the cephalothorax extending out, while the abdomen is folded 

 upon the body. Still later, July 1st, the trilobitic shape (figs. 24, 24 a, side view ; 24 h, 

 ventral view) is assumed, just previous to hatching. The trilobate character of the tergal 

 surface of the body is very marked, the cardiac, or median region of the body being con- 

 siderably raised above the sides, and on the abdomen the sutures do not run continuously 

 across the tergum, but are interi'upted at the edge of the cardiac region, thus making the 

 latter more distinct. The sutures between the segments are sinuate, as in the Trilobites, and 

 the eighth segment is parallel to the median line of the body, as well as the ninth, which 

 forms the rudimentary spine. The outer edge of each segment is convex, the convexity 

 greatest on the hinder portion of the edge, as in the Trilobites. The lateral region on the 

 cephalothorax is limited by the slight ridge on which the ocelli and compound eyes are situ- 

 ated, which may be called the opthalmic ridge. The green yolk is still lobulated along the 

 edge with six lobes on the lateral region, indicating the six cephalothoracic segments. The 

 sternal portion of the abdomen is now flattened, having been absorbed, so that on a side 

 view it cannot be seen, nor even the abdominal legs. These last are at first a little less 

 than one-half their size in the fully formed embryo (fig. 24), and the median sinus between 

 each leg is greater in the first pair than in the second. The rudimentary feet in the first, 

 or basal pair, form a minute tubercle (fig. 22) on the inner edge of each gill-bearer. 

 Those of the second pair are much larger, being long, slender, nonarticulated px'ocesses di- 

 rected obliquely outwards. The maxillipeds are much as in the larva, the coxal joint 

 having a stout spine directed inwards ; but it is not until after several moults that the 

 numerous spines which form the masticating surface appear. The claws are acute and 

 long. Fig. 23, a, b, represents the terminal joints of two of the anterior maxillipeds, and 

 23, d, the terminal joints of the fifth pair, the six spatulate spines on the penultimate joint 

 of the adult being now simple spines, and the terminal forceps being quite complete. Just 

 behind the last pair of maxillipeds are a pair of oval, rather flat, tubercles ; their bases are 

 united on the median line of the body, thence diverging outwards. These tubercles do not 

 appear much before this period in the life of the embryo. They have no connection with 

 the last pair of maxillipeds, nor can they be, as Savigny supposed, the rudiments of a 

 seventh pair of legs, otherwise they would have appeared earlier in embryonic life. It will 

 be seen by reference to figures 24 b, and 25 a, that they close up the posterior end of the 



