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MUSEUM OF COMPARATIVE ZOOLOGY. 189 
several authors about the rim of the disk and the margin of the oral lappets in 
the same species.* 
Two different sides may be distinguished in the larva. These may arbitrarily 
be known as the dorsal and ventral. The term dorsal as here used refers to 
the flexure of the body diametrically opposite that on which the drooping 
mouth hangs, while the mouth may be regarded as opening on the ventral side. 
In the imprisoned Nemertean there is also a corresponding dorsal and ventral 
side. The worm is fastened to the larval nurse by the ventral region, and is 
free from the amnion at all other points. It hangs in the amniotic cavity in 
such a manner that its ventral side lies in the same direction as the ventral 
side of the larva, and the proboscis extends into the recurved portion and lies 
in an extension of the amnion above the cesophagus. The posterior end of 
the body of the Nemertean in older stages of its growth is bent at right angles 
to its length, the extremity being bent upward on the ventral side. 
Fig. 2 represents the youngest larva of P. recurvatum as seen from the dorsal 
side. The proboscis is so drawn back that it does not inflate the upper pig- 
mented region of the amnion. The recurved outline of the upper part is 
turned away from the observer. The lower portion of the body is short and 
thick. The diameter from one side to the other is less than that measured 
dorso-ventrally. The general shape of the larva from this side is pyriform. 
One of the most prominent organs in the structure of the Nemertean enclosed 
in the Pilidium is a pair of spherical organs (cs), shown in both Figs. 1 and 2, 
just below the origin of the proboscis at its point of differentiation from the 
body of the worm. These bodies lie one on each side of a dorsal median 
line, and have lateral openings into the amniotic cavity in which the worm 
is contained, and are ciliated. They may be known as the cephalic sacs, 
and are probably the same as the “Saugnipfe” mentioned by J. Miiller. 
These organs are among the earliest structures to differentiate themselves in 
the growth of the worm, and in older stages of growth each opens externally 
on the sides of the head by a small ciliated orifice. Four of these bodies were 
mentioned by Miiller, and Biitschli speaks of and figures four in P. gyrans. 
Two only were seen in this stage of P. recurvatum. Another pair is of later 
growth. In the stages of growth older than Figs. 1, 2, the external shape of 
the larva is somewhat changed, but the increase in size of the Nemertean con- 
tained in and borne about by the free swimming nurse takes place without any 
changes of great importance in the external contour of the larva. 
A larva of P. recurvatum slightly older than that represented in Fig. 1 car- 
ries its snout in a very exceptional manner. In this larva that extremity of 
the body which is in the majority of cases simply dependent is carried pro- 
jected outward at right angles to the longer axis of the body. This mode of 
extending the mouth was observed in a single specimen, and may have been 
an individual peculiarity. It shows, however, the capabilities of movement 
which the snout has. 
The proboscis of a worm shown in Fig. 3 is very movable in the sac of the 
* Arch. f. Naturges., 1873, Band I. 
