16 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Paerurid zoeae, but I have not been able to find a trace of such a 
structure in the genus Eupagurus. 
In the face of the foregoing considerations, it certainly seems 
much more likely that the common structure of the young of Calli- 
axis and Naushonia is to be explained as due to relationship between 
their parents, rather than as a result of independent development in 
each species. The only direct evidence in favor of the latter view 
is the isolation of the two genera geographically. The zoea and 
mysis stages of Naushonia offer no peculiarity in their mode of life. 
The supposition of relationship of course makes no attempt to 
explain the origin of the unusual anatomical characters of the 
larvae, which is completely obscure, it merely assumes that they 
were received in both species as a legacy from common ancestors, 
in which they arose as an adaptation to a peculiar need, or perhaps 
merely as a variation which was not injurious and became prepotent, 
undisturbed by selective processes. In either event, the alterations 
when once fixed, might readily be handed down even if in the 
descendants they were without selective significance. The suppo¬ 
sition is also in perfect accord with the result obtained from com¬ 
parison of the parent forms. I am unable to give a description of 
C alii axis at first hand, since the species is rare and specimens could 
not be obtained. But the animal and many of its appendages were 
described with considerable detail by its discoverer, Heller (' 62 ), 
and these data are amplified and supplemented by the briefer 
accounts in the writings of Cano (' 91 ) and Claus (' 85 ). 
Comparison of these descriptions and figures with Naushonia 
shows an essential likeness between both species in almost all 
anatomical details, especially with respect to the carapace, abdomen, 
gills, and posterior mouth parts. Only the pereiopods and the 
appendages of the head are to any extent dissimilar. The carapace 
lacks the keels and supra-antennal spines and the external notch is 
weaker than in Naushonia. The triangular, serrate rostrum has a 
slight median furrow. The abdomen differs in the more acutely 
rounded epiinera, which are, however, procurrent, recurrent, or 
truncate, respectively, as in Naushonia, and in having the surface of 
the telson marked by a median furrow and two lateral keels. The 
gill formula is identical in both genera and the structure and rela¬ 
tive proportions of the slightly phyllobranchiate gills are the same, 
except that the European genus has the anterior gill on segment h 
smaller than its fellow, the mastigobranchs narrow and weak, espe- 
