104 



G. A. ELRINGTON 



we hâve no hésitation in admitting the identity of the form we hâve studied 

 with the larva described by the above mentioned authors. 



Then remain nevertheless certain points connected with the internai 

 organization of the larva, which hâve not as yet been recorded, and others 

 again which hâve been, as we believe, erroneously interpreted. 



The following observations hâve been made with the view of filling up 

 some of the gaps in our knowlcdge of the life history of one of our common 

 species of Polychœta. We are indebted to Professor G. Gilson for the spé- 

 cimens of this larva, which were obtained by him from the samples of 

 plankton collected during his expéditions in the southern part of the 

 North sea. 



GENERAL STRUCTURE. 



Extenialform. The gênerai structure of the larva of Lanice conclii- 

 lega is sufficiently well known, so that a brief description only is necessary 

 in order to show that our spécimens correspond to those already described 

 with more détail by other writers. 



The larvae measured on an average i mm. in length, and were com- 

 posed of about 30 segments. The body is cylindrical, the anterior région 

 being of greater diameter than the posterior, which tapers off to a blunt 

 point, and terminâtes in four papillse, in the centre of which is the anal 

 aperture. The position of the mouth is terminal. The prostomium bears a 

 row of tentacles varying in number from three to five, a condition which 

 apparently dépends upon the progress of development. The middle tentacle 

 is usually longer than the others. The ventral surface of each tentacle has a 

 ciliated longitudinal groove extending from the base to the extremity. 



The first 17 segments are furnished with bundles of fine capillary setse. 

 Thèse are not to be found on the posterior segments. Uncini are présent 

 on ail the segments, but differ in appearance in the anterior and posterior 

 régions of the body, those of the anterior région hâve the form and arrange- 

 ment characteristic of the adult worm, but in the posterior région they are 

 represented by a few small hooks placed at the extremity of a protubérance, 

 called by Claparede the ventral cirrus (Bauchcirrus). 



A pair of pigmented eyespots may be seen on the dorsal surface of the 

 head, and behind them, on either side of the first segment is an otocyst. 

 On examining sections through this part, we found that the otocysts com- 

 municate with the exterior by means of a narrow duct. 



