DEVELOPMENT OF LIMULUS POLYPHEMUS. 



163 



thinning out over the space between the sixth protozonite and the base of the abdomen, 

 and again becoming thinner over the anterior end of the head. It does not follow the con- 

 tour of the body, but passes straight over the limbs. Moreover, there are indications that 

 the superficial layer of the cells composing the blastoderm is peeling off, for at the suture 

 between the end of the abdomen and the yolk, there is a sUght rent (fig. 12 b, a) between 

 the embryo and the skin, and this increases with the age of the embryo, peeling off from 

 behind forwards. The appendages become more prominent, and in a day or two more ap- 

 pear as distinct tubercles. As the limbs grow longer they gradually push away the proto- 

 derm, the body of the embryo meanwhile becoming larger and outgrowing the blastoder- 

 mic skin, as seen in fig. 13, where the embryo is a conspicuous raised disk, and the limbs 

 form slender finger-shaped tubercles, with au external muscular layer, and within filled 

 with yolk granules. 



Fig. 15 shows the embryonic disk still thicker, while the yolk is more distinctly walled 

 in by the tergal extension of the body walls. I am disposed to compare the embryo at 

 this age, with the free swimming nauplius condition of other En- 

 tomostraca, especially of the Copepoda, though I am aware that 

 the differences are very great, and that this opinion may not be 

 shared by other observers. The adjoining cuts of the young of 

 Branchipus (a) and Apus (&) give a fair idea of a nauplius of the 

 Phyllopods, just after leaving the egg. The oval body, with the 

 abdomen not yet differentiated, and bearing no limbs ; the 

 (usually) three pairs of limbs, representing the first and second 

 antennae and mandibles of the adult, together with the single eye, 

 are the distinguishing characters of the nauplius stage. How 

 does our Limulus embryo compare with this? There are six pairs 

 of appendages, all arising from the head, and there are no ab- 

 dominal ones ; the abdomen is not yet separated from the body, 

 though there are faint indications of a separation of what is to be the abdomen, from the 

 anterior part of the body. Though there are in nauplius usually three pairs of appendages, 

 yet not uncommonly there are but two (being probably the first and second antennae). 

 Nordman^ figures the nauplius of several parasitic Copepoda which hatched with but two 

 pairs ; i. e., Achtheres percarum, Lernceocera cyprinacea, Tracheliastes p)olycolp)us ; while 

 in Ergasihis Sieholdii, belonging to another family of Siphonostomous Copepoda, the 

 larva is provided with three pairs of appendages, as in the larva3 of all the free swimming 

 Copepoda, and the Cirripedia. Ascending to the Cladocera we find that Dohrn ^ has figured 

 the nauplius stage of the embryo of Daphnia with three pairs of appendages, and it is in some 

 respects farther developed than the embryo of Limulus, in having at the same time a rudi- 

 mentary upper and under lip. In the Phyllopoda the larva of Lbnnadia Hermcmniis Rgured 

 by Lereboullet ^; there are but two pairs of appendages, which finally become the first and 

 second antennae, and an enormous lab rum is developed. The larva of Artemia salina * and 



' Micrographische Beitrage zur Naturgeschichte der 

 wirbellosen Thiere. Heft 2, 1832. 



''Untersiichungen Uber Ban uml Entwickelmig iler Arthro- 

 poden. Hfft 1, Taf. 1, fig. 8, 1870. 



* Observations sur la Generation et le Developi ement de la 



Limnadia Hermann! Ann. des Sc. Nat. 5"" Ser. Tom. 5, 

 186fi, pi. 12, figs. 1, 2. 



* Joly, Histoire d' Artemia salina, etc., Ann. des Sc. Nat. 

 2d series, Tom. 30, pi. 7, figs. 4, 5, 6. 



