306 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
DESCRIPTIONS OF GENERA AND SPECIES. 
I. DIURELLA Bory de St. Vincent. 
Generic characters . — Two toes, either equal, or the shorter more than one-thircl the length of 
the longer. The longer toe less than one-half the length of the body. Body nearly cylindrical; 
curved or twisted. 
• A. Descriptions of the Species studied by the Author. 
1. Diurella tigris Muller (pi. i, figs. 1-6). 
Synonyms: Trichoda tigris Muller (1186); Notommata tigris , Ehrenberg (1833, 1838); Heterognathus macrodactylus 
Sclimarda (1859); Monommata tigris , Bartscli (1870); Rattulus tigris , Hudson & Gosse (1889). 
Distinguishing characters.- — This species may be known by the two equal toes (fig. 6), the 
single tooth at the dorsal anterior edge of the lorica, and the nearly cylindrical curved body. It 
has a striking resemblance to D. tenuior Gosse, from which it is distinguished by the equality of 
the two toes. It differs also in the usually greater size, the somewhat greater prominence of the 
ridge, and the slightly greater slenderness and distinctness of the foot. 
External features. — The body is elongated and curved, appearing to be cylindrical, in a 
cursory view. Really the body rises to a ridge on the right side, so that in section it has the form 
shown in fig. 5. Preserved specimens usually lie, owing to the form of the body, in such a position 
that the ridge does not appear in profile, hence it is very easily overlooked; in living specimens it 
is more conspicuous. 
The head-sheath is rather distinctly set off from the remainder of the lorica by a constriction. 
It is marked by nine longitudinal plaits (fig. 3), at which the head-sheath folds when the head is 
withdrawn, thus closing the anterior opening completely (fig. 4), On the right side the anterior 
edge bears a single prominent tooth. 
The ridge (fig. 1) begins as a backward prolongation from the base of the tooth. It extends 
backward and to the left, seeming to have a slightly spiral course, and reaching almost to the foot. 
Along its left side are transverse striat.ions, similar to those so prominent in many species of 
Rattulus, but less conspicuous. 
The degree of development of the ridge varies greatly in preserved specimens. In some it can 
scarcely be seen at all. In others it is visible in the anterior part of the body, but seems to extend 
only half the length of the lorica or less. These differences are perhaps due only to optical diffi- 
culties resulting from the position of the specimen, but I am inclined to believe that there are 
really such differences in the development of the ridge in different specimens. These differences 
are perhaps functional, depending upon the degree of contraction of the animal (see the general 
account of the striated area, p. 281). In view of these facts the size and length of the ridge can not 
be considered a distinguishing character in this species. 
As a whole, the body may be seen to form a segment of a spiral, a spiral that is further accen- 
tuated by the position of the toes ( q . v.). 
Corona . — The corona bears a single club-shaped frontal process; its other features have not 
been studied especially. 
Antennae . — The dorsal antenna (fig. 1 , cl. a. ) lies just to the left of the ridge, a very little behind 
the constriction separating off the head-sheath. The lateral antennae are in the usual position, on 
the posterior third of the body (fig. 1). 
Foot . — The foot is rather slender and sharply set off from the body. The joint between the 
foot and body appears to lie in a transverse plane, without the asymmetry which is so marked in 
many of the species of Rattulus. 
Toes (fig. 6). — The two equal toes are stout, curved rods, about one-third the length of the 
body. They are attached to the foot in such a way that the base of the right toe lies above that of 
the left, and the concavity of the toes faces to the right (fig. 1). When the toes bend (at their 
attachment), they bend to the right. Each toe has at its base a number (at least four) of short, 
sharp spines or substyles (fig. 6) . At the base of each toe opens one of the two mucus reservoirs 
(fig. 6, m. r.). 
