190 BULLETIN OF THE 



amnion in which it lies. It is here represented as filling the whole sheath, 

 although it seldom remains long in this state, but is drawn back and forth as 

 if even at this early stage in its career its larval life was soon to terminate. 

 The walls of the cavity of the proboscis are well seen in a larva of the ao-e 

 shown in Fig. 2, especially when it is extended so as to fill its amniotic sac. 

 Fig. 4 is another larva of about the same age as that represented in Fig, 3, 

 which carries its snout turned down as in the majority of specimens cap- 

 tured. Fig. 5 is slightly older than Fig. 4, and is a view of the last from the 

 dorsal side. It has at the lower pole a short flagellum, smaller than that 

 found at the apex of the larva, but prominently larger than the majority of 

 vibratile cilia with which the whcjle external surface of the larval body is 

 covered. 



Figs. 6, 7, 8, represent stages in the development of the worm which show a 

 .progressive growth of the contained Nemertean. It will be noticed that the 

 whole larva has considerably lengthened its body and become slighter, while 

 the pigmented sac which encloses the proboscis has become much larger. It 

 is also to be noticed that the projecting snout upon which the mouth of the 

 nurse is situated has become contracted in size, and that, as shown in ¥i<'. 7 it 

 has dwindled to a slight prominence. The CEsophagus also keeps pace with 

 this reduction in size of the projection in the cavity of which it lies. 



In Fig. 9 the relative size of the " nurse " and the contained Nemertean is 

 very different from that shown in previous figures. The proboscis now forms 

 a large and prominent body in a pigmented amniotic sac, filling most of the 

 upper portion of the larva. It moves back and forth in a most restless manner 

 within its prison walls, and seems attempting to escape. The muscular threads 

 which formerly united the apex of the larva with the cavity have disappeared. 

 The lower end of the worm has gro\\ni so long that it is folded upward on 

 the ventral side of the nurse, reaching a short distance above the region of the 

 larva in Avhich the ring of cilia lies. The whole of the amnion in which 

 the posterior end of the worm lies is pigmented a fine dark red color similar 

 to that upon the proboscis. On the ventral side of the Nemertean there is 

 an enlargement which is the unabsorbed part of the contents of the amnion 

 transmitted from younger conditions of growth. The walls of the anmion 

 fit tightly upon the worm within, but in places they can be very easily 

 distinguished from those of the worm. The cephalic sac is well marked, 

 the cavitj^ of the proboscis clearly evident, and a well-marked organ on the 

 dorsal side of the Nemertean is probably the primitive formation of the dorsal 

 water-tubes. 



In Fig. 10 is represented the oldest Pilidium which we have observed. The 

 contained worm has outgrown its narrow confinement, and there remains one 

 important change by which it can extricate itself. If in order to hasten on 

 this change, somewhat akin to evisceration, the larva be put in a small quan- 

 tity of water, as in a watch crystal, the worm thus confined will be observed to 

 move in the amniotic cavity even more briskly than before, and to fret more 

 strongly against the barrier which envelops it. This hastens on the " critical 



