144 BRODE. [Vo. XIV. 
2. NATURAL HIsTory. 
The family Naidomorpha includes many minute transparent 
worms varying in length from 1 to 15 mm. Some members of 
the family may be found in almost every collection of water 
plants from a pond or ditch. 
Members of the genus Dero may be recognized at once by 
the presence of digitiform processes at the posterior end of the 
body (Pl. XIII, Fig. 2), and the most apparent distinguishing 
character of Dero vaga is its habit of building a case for itself 
and pulling it about on the surface of the water (Pl. XIII, 
Pig. 1): 
Specimens may be found in ditches, ponds, and small lakes 
where there is an abundance of vegetation. For this work col- 
lections were made at Glacialis Pond near Cambridge, Mass., 
and at Wolf Lake, Ill. The surface of the Cambridge pond 
was almost covered with Lemna, and the worms were remark- 
ably abundant. The worms prefer a pond in the open field, 
but are found most abundantly in the shade of leaves of water 
plants near shore. In case there is a lack of Lemna the worms 
may be found in algae below the surface or even on the bottom 
of the pond. 
Dero vaga varies in length from 5 to 10 mm., according to 
the progress of the growth preceding fission. The width never 
exceeds .25 mm. The number of segments may vary from 
twenty-five to sixty. All segments excepting the first five have 
four bundles of setae. The first segment (prostomium) has no 
setae. Segments II-V have ventral setae only. 
The anterior end of the body is slightly enlarged, and during 
locomotion the pharynx is everted to form a sucking disc. The 
branchial apparatus at the posterior end of the body is made 
up of a disc with undulating edges, which occupies a dorsal 
position, and two well-defined digitiform appendages which are 
found near the ventral side. In addition to these parts there 
are two long, slender, outwardly curving appendages with en- 
larged tips which arise below the other parts (Fig. 2). All 
parts of this apparatus are covered densely with cilia, which, 
during life, are in constant motion. When the worm is undis- 
