MUSEUM OF COMPARATIVE ZOOLOGY. 189 



oeveral 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 opiDosite 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 iti 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 oesophagus. 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 " Saugnapfe " 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. recurvation. 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 inode 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. 



