294 PROCEEDINGS OP THE AMERICAN ACADEMY. 



ring of the somite (ann. n.), :i structure typically found in annulua 2 

 ( see somite V 1 1 1., Fig. I ) . 



Somite \ I. clearly consists of three annuli, of which we can identify 

 :; and - by the dorsal sensillaa and the anterior nerve ring respectively. 

 In place of annuli 1 aud ■'< we have in Bomite VI. a Bingle annulua, in 

 which, however, Bristol found no nerve ring. 



Somite V. consists, apparently, of two annuli. But what the distri- 

 bution of nerve \ '." Bhows to be undoubtedly the anterior nerve ring of 

 this Bomite lies just within the limits of the uexi anterior annulus I IV.'. 

 left half of Fig. 4). Our first impulse would be to include thai annulus 

 in somite V.; but a careful study of the distribution of nerves I/'— IV." 

 shows that tin- annulus in question contains important Structures i includ- 

 ing the eye and associated mum- organs) belonging umpiestionabh to 

 somite I V. It is possible to suppose that in this case the most anterior 

 part of somite V. bas [used with somite IV. It stems to me, however, 

 more reasonable to explain the condition as illustrating a general 

 tendency in the head region for nerves t<> be carried forward of the 

 somites to which typically thev would be limited. For example, nerve 

 V." (Fig. 4) is carried forward ventrally as tar as the last annulus of 

 somite II.; nerve IV." runs into the anterior annulus of somite II.: 

 nerves II." and III." are carried forward into somite I. In Grlossiphonia 

 (Fig. 1) also a similar tendency can be recognized in the distribution of 

 nerves III."— V." The nerve ring found in the posterior portion of the 

 annulus assigned by Bristol to somite IV. (Fig. 4, left half) cannot 

 reasonably be considered the posterior nerve ring of somite IV.. because 

 it is connected exclusively with nerve V." (See Bristol's PI. VII., 

 Fig. 1G.) Moreover the posterior nerve ring has disappeared in Bomites 

 V. and VI., we should therefore expect to find it wanting also in the 

 more abbreviated somite IV Finally, unless this nerve ring does 

 belong to somite V., that -finite contains no nerve ring al all, and somite 

 IV., which is much more extensively abbreviated, still retains a nerve 

 ring. This seems very improbable, for we find that nietanieric structures 

 omitted from an abbreviated somite, do not appear in other BOmites still 

 more strongly abbreviated. 



In determining the external limits of somites I. -IV.. Bristol has 1" I D 

 guided by the position of the nietanieric BCD 86 organs and the distribution 

 of the nietanieric nerves I." -IV.". which represent the nerve- given off 

 from one side of primitive ganglia I.— IV. respectively, completely fused 

 into Bingle trunks. The somite limits indicated by him in the case of 

 these four metameres are. accordingly, neuromere limits, and, bo Far as I 



