192 BULLETIN OF THE 
as shown in Figs. 11, 12, and 13, has become reduced in size, so that now the 
greatest diameter is found just behind the cephalic sacs, and the body tapers 
from this point backward to its posterior end. The larva has three pigmented 
regions, viz. the very anterior end of the snout, the region just behind the 
cephalic sacs, and the posterior end of the body. The eye-spots are found on 
the sides of the head just in advance of the cephalic sacs. These last-men- 
tioned organs are relatively smaller than formerly, and are ciliated in their 
interior and on the inner walls of the tubes which lead to them from the 
external orifices. 
The internal organs differ slightly from those of the adult Lineus. A large 
stomach, the movements in the walls of which could be easily seen, fills most 
of the interior of the larva. There is in this larva no visible anus. The sinu- 
ous tube (wt) which lies on the dorsal side of the stomach has been identified 
as a water vessel. A similar organ is figured by Leuckart and Pagenstecher in 
P. gyrans. 
The form of this singular larva and the strange development of the enclosed 
worm suggest very interesting theoretical questions. The external outlines of 
the larva ally it to animals widely separated in our classification from the 
Nemertean to which it gives origin. Busch, Leuckart and Pagenstecher, and 
others, have pointed out that in the similar relationship of Nemertes to its 
Pilidium we have a parallel condition to that which exists between an Echinus 
and its pluteus. In the species of Pilidium which they studied, not only the 
stomach of the larva, but also its esophagus and mouth, were found to be 
directly changed into the same organs in the contained Nemertean. In P. re- 
curvatum the resemblance which they suggested is even closer than in those 
species which they have studied, for here not only is the larval mouth and 
cesophagus only indirectly if at all changed into the same organ in the adult, 
but also the lower portion of the embryo has a true brachiolarian form which 
is highly suggestive. Our larva, even more than those of other species of Pili- 
dium, shares with Tornaria and Actinotrocha many Echinoderm characteristics. 
The history of the opinions which have been advanced by Miiller, Busch, 
Leuckart and Pagenstecher, Huxley, A. Agassiz, and others, in relation to 
the resemblance or want of likeness of Tornaria to the young Echinoderms, 
is too well known to be repeated here. If Balanoglossus were the only worm 
whose larva resembles the young Holothurian, the fact might be explained by 
the abnormal character of the adult. With the Nemerteans, however, the case 
is somewhat different, for in them we have a large group, whose larve have 
many points of resemblance to the embryonic Echinoderm. Nowhere is that 
likeness carried so far as in the strange Pilidiwm recurvatum, which has been 
described above. It would seem at first sight that the circular belt of cilia 
described in this larva would be an argument against its close affinity with the 
larval Echinoderm. The same thing may be said of this, which has already 
been said of a similar belt in Tornaria. Far from being an unknown feature, 
it is a peculiarity in some Echinoderm young, as in Comatula and the Holo- 
thurians, which are the closest allies of the worms. We find in some young 
