110 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Dana. All of the members of the Sididae thus far known are 
marine and especially distinguished from all other families of the 
Cladocera by the presence of six pairs of thoracic appendages and 
by the fact that one or both of the branches of the second antennae 
have less than three joints, the characteristic number for most of 
the members of this group. The species has been determined by 
Prof. E. A. Birge of Madison, Wis., who has compared the form 
with the type specimens of M. Richard. 
The embryology of no member of the Sididae has been studied, 
and most of the work on the entire group has been of a sj^stematic 
or anatomical character. Only three papers of importance dealing 
with the embryology of the Cladocera are known to me; these are 
by Grobben (’79), Lebedinsky (’91), and Samassa (’93). 
Other articles, such as those of Dohrn (’69) and of Zaddach 
(’41), are old and give but few details of development. 
Anatomy. 
Penilia when alive is transparent and has no pigment of any kind 
except the very small amount in its single median eye. It measures 
from 0.8 to 1.0 mm. in length, not, including the biramose swim¬ 
ming antennae, which with their setae are about 0.8 mm. The 
long pair of abdominal setae are 0.4 mm. in length, making the 
entire animal from 2.0 to 2.2 mm. long. In general shape (Fig. 1) 
it somewhat resembles Daphnia similis (Claus). At one stage 
of the life of Penilia eleven pairs of appendages appear, but in the 
adult ten only are present. The small anterior, or first, antennae 
are situated on the posterior side of the two prominent horns which 
project downward from either side of the head. They contain 
nerve cells, and one long sensory seta is attached to the anterior 
side of their distal end and to the posterior side a pencil of small 
sensory hairs. The large biramose swimming antenna is composed 
of one large basal joint from which extend an exopodite and endo- 
podite, each composed of two segments, the first about three times 
as long as the second joint. These joints have long, stiff setae to 
aid in swimming. The mandible is strongly developed and moved 
by comparatively large muscles. The first maxilla is composed of 
a single joint. It is situated directly behind the mandible and is 
much weaker than that appendage. It possesses setae on its end. 
