A STUDY IN MORPHOLOGY. 117 



but a comparison of these forms with the metamorphosis of Euphausia, upon which 

 GLAUS lays especial emphasis, seems to demand a directly opposite interpretation. 



If the Zoea has been produced by a secondary modification of the Protozoea we 

 should expect to find the characteristics of the Protozoea better preserved in the 

 Schizopods than in the lower Decapods, and if we find in the Schizopods certain 

 features of the typical Zoea, which are absent in the Protozoea of the lower Macroura, 

 we can hardly accept without question the interpretation which sees, in secondary 

 modification of the latter, the origin of the Zoea. In Euphausia the somites appear 

 in regular succession, from in front backwards, but the somites of the abdomen 

 acquire appendages before the pereiopods appear, and there is a stage when the 

 abdomen is fully developed and the thorax almost absent ; a stage which, therefore, 

 resembles the Brachyuran Zoea more perfectly than any stage in the development 

 of Lucifer, Acetes, Seryestes, or Penceus. 



We have no complete history of any one species of Euphausia, but the observations 

 of METSCHNICKOFF (Zeit. f. Wiss. Zool., xix., pp. 479-481, and xxi., pp. 390-401), 

 and GLAUS (Zeit. f. Wiss. Zool., xiii., pp. 442-454, and " Untersuchungen," &c., 

 pp. 9 and 33) give us a tolerably complete account of the metamorphosis of the 

 genus. 



METSCHNICKOFF'S larva is extremely like that of Lucifer, although there are many 

 differences. It is interesting to note that it leaves the egg in a much more rudi- 

 mentary form, passes through a greater number of moults, and attains to much greater 

 structural complexity than Lucifer during the Nauplius stage. We can select 

 three stages which agree pretty closely with the egg Nauplius, the first free Nauplius 

 and the last, or mein-Nauplius, of Lucifer, but between, after, and before these stages 

 there are others which are not found in Lucifer. 



The youngest Nauplius (Zeit. Zool. xxi., fig. 2) is so much less advanced than the 

 egg Nauplius of Lucifer six hours before hatching, that it does not seem probable that 

 it normally leaves the egg in this condition. 



It has an oval body, without ocellus, mouth, or labrum, and there is no trace of 

 more than three pairs of appendages or of the carapace. At the next stage the 

 swimming hairs of the first three pairs of appendages are fully developed, and the 

 anus, notch, and two spines of the telson are present. In these respects it is more 

 advanced, but in the rudimentary condition of the labrum and metastoma less 

 advanced, than the first free Nauplius of Lucifer. The buds for the first and second 

 maxillae and the first pair of maxillipeds are present, but continuous across the median 

 line of the body. According to METSCHNICKOFF, the larva shown in fig. 3 of his first 

 paper is in the next stage of development ; but I can scarcely believe that it belongs 

 to the same species, for the ocellus is absent, and the hairs on the three pairs of 

 locomotor appendages are much more rudimentary than they are in fig. 3 of the second 

 piipa. 



The next stage (fig. 4) of the second paper agrees with the first free Nauplius of 



