188 BULLETIN OF THE 
and their outer surface is ciliated. The lower hemisphere at a short distance 
from the lower pole is girt about by a ring of large cilia, which by their con- 
stant movement impart onward and various rotary motions to the embryo. It 
sometimes moves forward in the line of its length, and then whirls on its axis 
without any direct forward motion. Both of these movements are the results 
of ciliary action. From the thin outer wall to the cavity* within extend many 
muscular fibres, which are sometimes simple and sometimes compound, and 
are generally disconnected with each other. Two of these muscular threads 
are more prominent than the rest, and extend from a thickening at the apex of 
the larva to the junction of the cesophagus and stomach. These are regarded 
as homologous to those muscular strings in P. gyrans, which were long ago 
noticed by J. Miiller, and regarded by him, and later by Metschnikoff, as 
nervous elements. 
From the apical thickening of the walls of the larva there arises a short, 
flexible flagellum, which waves back and forth as the larva moves through the 
water. The interior of the larva is occupied by an cesophagus, and an amniotic 
cavity which contains a growing Nemertine worm. The csophagus fills 
almost the whole of the bent portion of the larva under the apex. It opens 
externally by a mouth with ciliated lips. Internally it is continued into the 
intestinal cavity of the Nemertean. Its walls are muscular, ciliated inter- 
nally, and contractile. The external lips are slightly pigmented. No intestine 
or anal opening was seen in the larva. The interior of the body, from the 
inner end of the esophagus to the walls which form the lower pole below the 
ring of cilia, is taken up by a sac, which has been homologized with the am- 
nion of P. gyrans. In this sac is formed the young worm. The most con- 
spicuous regions of the amnion are the upper, which is a prolongation toward 
the apex from the vicinity of the inner terminus of the cesophagus, and the 
lower part, near the anal pole, which occupies most of the body of the larva. 
Both of these regions have the walls of the amnion thickly pigmented, as 
shown in the figures. In the blind sac which constitutes the upper of these 
pigmented regions lies the future proboscis of the worm. This last structure 
is movable in the pigmented sheath in which it lies. It sometimes completely 
fills its sac, and when withdrawn leaves the pigmented amnion in the shrunken 
condition shown in the figure. The pigmented regions are composed of small 
granules of a dark red color closely crowded together. They are represented in 
Biitschli’s figure t of P. gyrans by a single large and irregular pigment spot. 
This amniotic pigmentation is not the same as the colored bodies described by 
* An Amnion such as has been described in P. gyrans is already formed in the 
youngest larva of P. recurvatum which was taken. 
+ In the young of Polygordius (Loven’s larva), we have described around the 
margin of the disk a number of problematical bodies, which are very similar to 
those spoken of by several authors as existing on the rim of Piidixm. In both 
genera they may be foreign bodies, and not patterns of pigmentation. In some 
specimens of a large undescribed Pilidium, found at Newport, they were present; 
in others, apparently of the same species, absent. 
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