MUSEUM OF COMPAEATIVE ZOOLOGY. 181 



ciliated on its outer walls. The ridge upon which the large cilia arise is col- 

 ored light red. At the lower pole near the vent there is a ring of small cilia. 

 No segmentation has yet appeared in the lower hemisphere. There are con- 

 spicuous pigment spots in the lateral walls of the prseoral lobe, but they appear 

 in irregular patches of red and brown color. The general disposition of the 

 internal organs is easily seen through the transparent body walls. By far the 

 greater part of the interior of the larva is taken up by the stomach (s), a globular 

 inflated sac with gall-green colored walls. It occupies most of the interior of 

 the oral lobe of the larva above and in advance of the ciliated equator. The 

 digestive tract communicates with the external water through two orifices. 

 The first of these is a mouth, and the vestibule intermediate between it and 

 the stomach is j^rrobably the oesophagus. The mouth opening lies on one side 

 (ventral) of the larva, just below its ciliated equator. The lower lip of the 

 mouth is richly ciliated. The double row of cilia which together make up the 

 prominent mesial ring divides in the vicinity of the oral opening, one half, 

 composed of smaller cilia, passing below the mouth on the lower lip ; the 

 other, or larger, skirting the border of the upper lip. The inner walls of the 

 oesophagus and stomach are likewise ciliated. The lower pole of the larva is 

 placed at an imequal distance from the equator measured on ventral and dorsal 

 sides of the body. Almost the whole of the lower part of the cavity of the 

 embryo, below the plane of the equatorial band of cilia, is taken up by the 

 intestine, which is an elongated sac-like body opening into the stomach on one 

 side, and into the external water through the vent on the other. 



The pattern (Fig. 1, a) of color on the anal pole of the larva is characteristic 

 in Nephthys, and on that account has been of great service in identifications of 

 larviB of different ages, whenever one could not be raised from another. The 

 pigment is here arranged as follows. Two small areas of green color are found 

 on either side of the anal pole. These areas coalesce with each other on the 

 dorsal, and are distinct on the ventral side. Slightly in advance, and removed 

 from them by a colorless zone, there is a narrow parallel band of green, closed 

 on the dorsal, and open on its ventral side, encircling the body. This imclosed 

 band marks the position of an anal ring of vibratile cilia. It persists with the 

 same characteristic form in very late stages in the development of the worm. 



The next oldest larva (Fig. 2) to that described has assumed a slightly dif- 

 ferent form from the last. The body is no longer spheroidal, but the lower 

 hemisphere has elongated to double its original length. In this growth the 

 prseoral lobe has taken, comparatively speaking, no share, and still remain? 

 of about the same shape as before. A zone of green appears about the pole of 

 the prteoral lobe, and most of the scattered cilia on the external surface of the 

 body have disappeared. The lower or body hemisphere, on the dther hand, 

 has become elongated and segmented, although no parapodia have yet ap- 

 peared in the several body segments. The intestine has lengthened con- 

 siderably. Its walls, as well as those of the stomach, have a green color, as in 

 younger larvae. 



In a larva still older (Fig. 4) the growth of the body hemisphere has gone 



