322 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Internal organs. — These offer nothing of unusual interest. The stomach is usually large and 
very opaque, making it difficult to study the internal structure. The eye lies considerably to the 
left of the ridge. The jaws are rather weak and are unsymmetrical, the right manubrium 
being a short, very slender rod, while the left one is stout and very much larger. 
Measurements. — Length of lorica, without toes, 0.17 mm.; of longest toe, 0.08 mm.; of shorter 
toe, 0.03 mm.; total length, 0.25 mm. 
Movements. — Rattulus gracilis Tessin is a slow swimmer. As it moves through the water it 
revolves upon its long axis to the right, so that the path becomes a spiral. The dorso-dextral ridge 
is always directed toward the outside of the spiral. In other words, the animal swerves contin- 
ually toward the ridge, the latter serving thus to cut the water. When stimulated suddenly, as 
by coming in contact with an obstacle, the animal swerves strongly toward the dorso-dextral side — 
that is, toward the ridge. 
A specimen of this species was seen to feed upon a young specimen of Diurella tenuior Gosse. 
The jaws of Rattulus gracilis Tessin were extended far out (as in fig. 49) and seized the side of the 
prey; a piece of the Diurella. was then torn out and devoured. 
History. — This species was first described by Tessin (1886) under the name of Acanthodactylus 
gracilis. In 1889 Gosse described the same animal in the supplement to Hudson & Gosse’s Mono- 
graph under the name Mastigocerca iernis. Bilfinger (1894) has given a better description of this 
animal than either Tessin or Gosse, but did not give a figure. 
Distribution. — In America: East Harbor, Lake Erie; Graveyard Pond, on Presque Isle, near 
Erie. Pa.; Huron River at Ann Arbor, Mich, (abundant in Ceratophyllum) . 
In Europe: Hear Rostock, Germany (Tessin, 1886); lakes in England (Gosse, 1889); Ireland 
(Hood, 1895): Wurttemberg, Germany (Bilfinger. 1894). 
19. Rattulus scipio Gosse (pi. v, figs. 50-52; pi. xiii, figs. 111-112). 
Synonyms: Mastigocerca scipio Gosse (1889); Mastigocerca unidens Stenroos (?) (1898); Mastigocerca cuspidata 
Stenroos (?) (1898). 
Distinguishing characters. — This species is distinguished by the usually somewhat prismatic 
lorica, sometimes curved, widely open in front, with the head-sheath not sharply set off from the 
rest of the lorica; the single tooth near the anterior margin, and the single long toe three-fourths 
or more of the length of the lorica, accompanied by a short “ substyle ” (the right toe). 
External features. — The lorica is elongated, often with nearly straight sides (fig. 50), though 
sometimes curved (fig. 111). In adult specimens (figs. 50 and 52) the diameter of the body is 
nearly uniform for three-fourths of the length, being very little narrower in the head region, and 
at the posterior end tapering in conical fashion to the foot. In young specimens (fig. 51) the 
largest part of the body is nearer the anterior end, and the lorica tapers thence regularly backward 
to the foot. The form of the lorica is not greatly changed in fully retracted specimens. 
Considerably to the right of the middle line the lorica rises to a pronounced dorsal ridge, 
which aids much in giving the body a prismatic appearance. The ridge inclines sharply to the 
right and extends from the anterior edge fully three-fourths of the length of the body. It is 
marked with the usual transverse striations; these extend for a considerable distance to the left of 
the ridge (fig. 52). At the anterior end the ridge bears a tooth, which is fairly prominent though 
not large. The tooth is not at the very anterior margin of the lorica, but arises from a little 
behind this. In a retracted specimen (fig 52) it projects slightly beyond the edge of the lorica, 
while in extended specimens (fig. 51) its tip may not reach the edge. 
The head-sheath is not very sharply marked off from the rest of the lorica, though a slight 
constriction between the two may be detected, especially marked on the ventral side. The liead- 
sheatli does not show longitudinal folds or flutings, such as are prominent in Rattulus longiseta 
Schrank and R. gracilis Tessin, and does not constrict or change its form greatly when the head 
is fully retracted. This gives one of the most striking characteristics of this species. At the 
anterior edge the head-sheath flares a little (fig. 51) and the anterior aperture remains widely 
open, even when the head is retracted (fig. 52). 
Corona. — The corona bears a thick, in dorsal view somewhat triangular, dorsal process. 
Antennce. — The dorsal antenna lies to the left of the ridge, at left edge of striated area. The 
two lateral antenna? are in the usual position, the right one being a little in advance of the left. 
