MUSEUM OF COMPARATIVE ZOOLOGY. 183 



longer, and additional segments have formed in the body, by which a still 

 greater likeness to the adult worm has been acquired. It is still, however, 

 free-swimming, although sometimes resting iipon the bottom of the glass in 

 which it is confined. The penultimate as well as the terminal body segments, 

 are destitute of spines. The oesophagus has lengthened considerably, and 

 throuf^h its walls the jaws (j) can be easily seen between the third and fourth 

 segments.* The stomach walls are opaque and colored green. 



The " pigment dots " {k) found on the apex of the prseoral lobe have now 

 changed their position in the head, and moved somewhat backward towards 

 the middle of the body. Each lies on the outside border of a transparent 

 body of spherical shape, which touches on the median line of the larva a 

 similar body on the opposite side. Both are situated in the dorsal walls, 

 and are almost invisible when the larva is viewed from the ventral side. The 

 rows of cilia about the neck are very active, as are also those near the posterior 

 end of the body. The head retains its comparatively large size, and is without 

 appendages. 



The next stage (Fig. 9) in the development of Nephthys shows us one where 

 the reduction in the relative size of the head and body has gone on, and the 

 cilia, which once formed such a prominent feature about the lower part of the 

 former, have almost wholly disappeared. The head bears a single pair of short 

 antennae. The body is composed of ten setiferous parapodia and a pair of ter- 

 minal somites, which are without spines. The segments are separated by deep 

 constrictions, and each parapodium is composed of a ventral and dorsal pro- 

 tuberance, both of which bear serrated spines. The ventral cirrus is short 

 and blunt ; the dorsal long and slender. 



The oesophagus has elongated to such an extent that its posterior end now ex- 

 tends backward in the body cavity to the eighth segment. The " eye-spots " (k) 

 lie in the dorsal walls of the third body segment. When the oesophagus is 

 protruded outside the mouth opening, these bodies retain their relative position 

 as regards the segment, and are not moved with it, which indicates that they 

 are not connected with the digestive tract, as might at first be supposed. The 

 mandibles, with which they might be confounded, lie between the sixth and 

 seventh segments, and can be protruded with the proboscis. 



The oldest larval Nephthys (Figs. 11, 12) which was observed has ten body 

 segments which bear spines. I was able in one specimen to detect on the head 

 the beginning of a single representative of a second pair of antennae, although 

 such could be seen only on one side. In this larva, which is shown in Fig. 11, 

 the head is quite small as compared with the body. The distance from the tip 

 of one lateral spine on a body segment to the end of another, on the opposite side 

 of the body, is three or four times the breadth of the middle of the body. The 

 dorsal region of the head has a green color, in which are irregular patches of 

 black and red. The black pigment probably later concentrates into those 



* In the oldest Nephthys, figured by Claparede and Metschnikoff, they lie oppo- 

 site the first body segment. 



