188 BULLETIN OF THE 



and tlieir 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 musculai threads 

 are more prominent than the rest, and extend from a thickening at the apex of 

 the larva to the junction of the oesophagus 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 oesophagus, and an amniotic 

 cavity which contains a growing Nemertine worm. The oesophagus 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, i'rom the 

 inner end of the oesophagus 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 oesophagus, 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 f 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 Amnio7i such as has been described in P. ijyrans is already formed in the 

 youngest larva of P. recurvatum winch was taken. 



t In the young of Polugordius (Loven's larva), we have described around the 

 margin of tlie disk a number of problematical bodies, which are very similar -to 

 those spokjen of by several authors as existing on the rim of PilidiKm. In botii 

 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. 



