6 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
while as a rule, phyllobranchiate filaments among the Thalassinidea 
are associated with reduction in the number of the gills, especially 
the ventral rows, and also pleurobranchs might be expected in a 
species with so large a number of gills. The internal anatomy 
unfortunately cannot be determined until more specimens have 
been collected, and we do not know whether peculiarities are 
present in other regions beside the exoskeleton and the appendages. 
Preadolescent Development. 
The study of the development even more than that of the anat¬ 
omy tends to separate Nauslionia from other Thalassinids, although 
it does not suggest a relationship to any remaining Crustacean 
group. 
The zoea and mysis-stage larvae (pi. 2, fig. 20-23) resemble in 
habit the young of other Macrura. They are positively phototactic, 
swim on their backs with a steady motion and come to rest with 
the head downward, but their slender form and ruddy color render 
them immediately noticeable. A closer examination shows that 
they have a body form very unlike that of most Decapod larvae, 
and that this is but little altered at the successive ecdyses. There¬ 
fore the five stages of * the preadolescent period may best be 
regarded as a unit, since zoea and mysis phases 1 cannot be satis¬ 
factorily delimited. The five stages can be differentiated as 
follows: — 
1. Third maxilliped rudimentary. Length, 2.2-2.6 mm. 
2. Third maxilliped with exopod functional as swimming organ; 
pereiopod rudiments noticeable. Length, 4 mm. (pi. 2, fig. 20). 
3. First pereiopod with exopod functional as a swimming organ ; 
remaining pereiopods rudimentary. Uropods first present. Length, 
over 5 mm. (pi. 2, fig. 22). 
4. Four anterior pereiopods functional as swimming feet; rudi¬ 
mentary pleopods present. Length, 7-9 mm. (pi. 2, figs. 21, 24). 
1 To avoid the ambiguity caused by the use of the word “stage ” to indicate 
not only the individual inter-ecdysal periods, but also the more comprehen¬ 
sive changes in form, in this paper the term will be confined to the former 
meaning. The more general change periods will be designated by the word 
“phase.” •. 
