1G4 A. S. PACKAKD, JR., ON THE 



Branchipus, and probably all the Branchiopodaj, have three pairs of limbs, which become 

 the two pairs of antennse and mandibles of the adult. Zuddach-' figures the larva of 

 Apus cancriformis as having but two pairs of appendages, which finally become the two 

 pairs of antennce of the adult. The nauplius stage of Nebalia, according to Metznikow's 

 figures, as in that of Limulus, is passed within the egg ; it has three pairs of limbs, and 

 rudiments of the abdomen appear. It appears, then, that the number of nauplian appendages 

 in the Entomostraca is inconstant, and though the embryo of Limulus has so great a num- 

 ber as six pairs, they are all cephalic and therefore homologous in their regional position to 

 those of nauplius-like larvae. No eye spots can be detected to compare with the single eye 

 of most nauplii, but no eye is developed in Daphnia, according to Dohrn's observations, 

 and indeed the only organ of vision, the compound eye, is developed late in embryonic 

 life ; neither is there in Leptodora according to P. E. Miiller. The nauplian stage of 

 Nebalia is eyeless. 



We feel justified, then, in regarding this stage of Limulus as representing, though im- 

 perfectly, the nauplius condition of the Entomostraca. Thus Limulus may possibly trace its 

 ancestry back to an ancestral nauplius-like being ; and as the number of limbs may have 

 been increased in the progress of development of the Limulus form, we can readily imagine 

 that the ancestral forms at the time when they were hatched as nauplii and not as sub- 

 zoese, had no more than perhaps three pairs of limbs, their growth in the egg being then 

 greatly accelerated to allow of their hatching in the nauplius stage, in contradistinction to 

 their present retardation in the egg until the form of the adult subzoeal Limulus is attained. 



The suhzoea stage. The next stage of the embryo is signalized by the appearance of 

 the two basal pairs of abdominal appendages (fig. 16, ah), with an accompanying enlarge- 

 ment of the abdomen. The arthromeres are also indicated beyond the areola, the sutures 

 extending upwards but not reaching the dorsal side of the egg. These changes occur some 

 time before the splitting apart of the chorion. The abdomen having grown a little longer 

 (the embryo now being about three-fourths the length of the yolk), two oblique, pale bands, 

 being local thickenings of the germ, can be detected on each side of the median line of the 

 body, and just behind the last pair of limbs. The cephalic feet have meanwhile increased in 

 length and are folded upon themselves. 



At a still later stage the embi-yo, observed June 21st, covers more than one-half of that 

 portion of the yolk in sight, and there are faint indications of the four median sutures on 

 the tergal surface of the yolk, showing that the yolk is surrounded by the subzoeal skin. 

 The areola is still well marked. The limbs are longer, but no joints have appeared, and 

 the rudiments of the foliaceous abdominal legs have become more apparent. In the same 

 egg, observed the next day, June 22d (fig. 17), the whole body has grown much wider, and 

 especially the abdomen. The sutures between the segments are much deeper and broader, 

 the legs have become three-jointed, two sutures appearing. The end of the abdomen is 

 broad and there are but two arthromeres, exclusive of the terminal one. In a side view 

 (fig. 17 a) the increased size of the limbs is shown and the larger size of the basal abdomi- 

 nal arthromere, or rudimentary gill-foot, as compared with the second one. The embryo 

 now revolves within the protoderm, which has somewhat increased in size, leaving a 

 greater space than before between it and the embryo, for its rotatory movements. 



^ De Apodis cancriformis, etc. PI. 4, fig. 3. 



