290 



IMMM'KI iMNi.- OF THE AMERK AN \< Mm Ml 



Whitman's ('92) theory that all the Bomitea of the body are of equal 

 morphologica] value becomes untenable. For, suppose the bodj to eon- 

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

 (the hypothetical primitive condition realized in all unabbreviated so- 

 mites). Now if the Bomite limits be marked nil so as to include in a 

 somite a Bensory ring and the two rings following it, a- is the practice of 

 Whitman ami others (Fig. •'!, left halt, compare Fig. 1. left half), we 

 shall have the absurdity of a ring at the anterior end of the bodj beloi 

 ing to /<"/"■ of the thirty-four somites, ami Bomite XXXIV. at the post* - 



rior end of the body will contain only tlOO rings. On the Other hand, it' 



neuromere limits and somite limits 

 are regarded as coinciding, all somites 

 of the body are of « iqua 1 morpholo° i- 

 cal value, each somite consisting of 

 three rings innervated from a si: 

 ganglion (Fig. ;! - 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 

 determining somite limits, what oth- 

 er criterion can we find ? Vaillant 

 ('70) suggested a search for septa. 

 But septa such as are found in Chse- 

 topoda are wanting in the leech, 

 unless we so regard (he entire blocks 



of mesodermal tissue which lie be- 

 tween the transverse lymph vessels. 

 The position of the lymph vessels is 



XXX 



XXXttl.' 



XXXIV.' 



XXXII. 



XXXIII. 



XXXIV. 



Fig. 3. Diagram showing tlic relation indicated by Stippling in the left half 



between neuromerea and somites in the f Figure 1 in the ease of Glos- 



i„„iy of a leech with three-ringed so- aphonia parasitica Say (Clepsine 



mites. — at the left of the diagram , ,, . ..... N _, 



. ' vm » r » »i ■ r hoUensts Whitman). 1 he transverse 



1 1 \ \ \I\ i, according to the view of 



Whitman; at the right (I.-XXXIV.), lymph vessels are found regularly in 

 according to the writer's view. the sensory ring of each Bomite ( r ig. 



1 , >•((. f.). It is generally admitted 

 that the lymph vessels represent a greatly reduced eel, mi. The blocks 

 of tissue between the transverse vessels arc. then, morphologically equiv- 

 alent to the Bepta of Cbaetopoda. But the middle of each such block falls 



