290 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Whitmau's ('92) theory that all the somites of the body are of equal 

 morphological value becomes untenable. For, suppose the body to con- 

 tain thirty-four distinct ganglia, each innervating three separate rings 

 (the hypothetical primitive condition realized in all unabbreviated so- 

 mites). Now if the somite limits be marked off so as to include in a 

 somite a sensory ring and the two rings following it, as is the practice of 

 Whitman and others (Fig. 3, left half, compare P'ig. 1, left half), we 

 shall have the absurdity of a ring at the anterior end of the body belong- 

 ing to none of the thirty-four somites, and somite XXXIV. at the poste- 

 rior end of the body will contain only two rings. On the other hand, if 



neuromere limits and somite limits 

 are regarded as coinciding, all somites 

 of the body are of equal morphologi- 

 cal value, each somite consisting of 

 three rings innervated from a single 

 ganglion (Fig. 3, right half). 



2. Septa and other Metameric Organs 

 as Criteria of Somite Limits. 



If we are unwilling to be guided 

 solely by the nervous system in 

 determininor somite limits, what oth- 

 er criterion can we find ? Vaillant 

 ("70) suggested a search for septa. 

 But septa such as are found in Chte- 

 topoda are wanting in the leech, 

 unless we so regard the entire blocks 

 of mesodermal tissue which lie be- 

 tween the transverse lymph vessels. 

 The position of tlie lymph vessels is 

 indicated by stipfding in the left half 

 of Fi<rui"e 1 in the case of Glos- 

 siphon! a parasitica Say {Clepsine 

 hollensis Whitman). The transverse 

 lymph vessels are found regularly in 

 the sensory ring of each somite (Fig. 

 1, va. t.). It is generally admitted 

 that the lymph vessels represent a greatly reduced coelom. The blocks 

 of tissue between the transverse vessels are, then, morphologically equiv- 

 alent to the septa of Chaetopoda. But the middle of each such block falls 



XXXI 



XXXIt! 



XXXIV/ 



XXXII. 



-1 XXXIII. 



XXXIV. 



Fig. 3. Diagram showing the relation 

 between neuromeres and somites in the 

 body of a leecli with three-ringed so- 

 mites, — at the left of the diagram 

 (I '-XXXIV.'), according to the view of 

 Whitman; at the right (I.-XXXIV.), 

 according to the writer's view. 



