211? Mr. A. Agassiz on the Young Stages 



passage from the stage represented by Loven in his iig. 5 to 

 that of his fig. 6. Although the body of the young worm is 

 much elongated^ the number of rings (fig. 9) has not greatly 

 increased ; they are further apart, and there is a tendency in 

 the stomach (which occupies nearly the whole width of the 

 body) to become folded, so as to correspond to the articula- 

 tions ; the anterior part of the head has greatly elongated, and 

 the general appearance of the young worm reminds us some- 

 what of the larva of Sipunculus nudus figured by Keferstein 

 and Ehlers. The vibratile rings are greatly reduced, the an- 

 tennae have slightly increased in length, and the head of the 

 worm presents a certain resemblance to a Nereid or some allied 

 form. The swelling of the posterior extremity has also been re- 

 duced, and the anal vibratile chord scarcely projects beyond 

 the line of the body. The pigment spots of the rings have 

 diminished in number, but shghtly increased in size ; and the 

 brilliant row of spots of the oral and anal rings is beginning 

 to fade, the vibratile cilia are losing much of their activity, 

 and the little worm, though still capable of swimming freely 

 about, and often caught at this stage with the dip net, moves 

 quite slowly and has gradually lost, with the extension of the 

 posterior part of the body, the rapidity of motion it enjoyed 

 in the earlier stages (figs. 3, 4). When kept in confinement 

 they are often found at the bottom of the vessel coiled up, 

 and when disturbed creep slowly away by undulations of the 

 body, assisted by the remnants of the vibratile rings. In a 

 somewhat more advanced stage (fig. 10) the pigment spots have 

 further diminished in size as well as number, the convolutions 

 of the digestive cavity are more distinct, the antennae have de- 

 creased in length, and the vibratile rings have lost their former 

 power. In a subsequent stage (fig. 11) the head has become 

 more distinct, the anterior vibratile ring scarcely exceeds the 

 diameter of the body, and the antennae are quite prominent. 

 The little worm is only rarely fished up in this stage, swimming 

 about very slowly, and becoming somewhat more active when 

 creeping upon the bottom, where they now prefer to remain. 

 This is their most advanced nomadic stage; and, from their 

 subsequent habits, it is necessary to keep them in confinement 

 in order to follow their later changes. 



We find in fig. 11 the pigment spots becoming smaller than 

 in preceding stages : the convolutions of the digestive cavity, 

 which has acquired a light yellowish colouring, are extremely 

 well defined. Up to this time we have still no trace of feet, 

 bristles, or appendages of any sort, except the two tentacles of 

 the head ; and were it not for these, it would seem as if the 

 young worm were the larva of some Nemertes-\\kQ animal, not- 



