302 PROCEEDINGS OF THE AMERICAN &< LDEMY. 



takes place, therefore, only by elongation of the individual Borail 

 which compose it. This method ol growth is probably what has called 

 forth the phenomenon of ring multiplication. In support of this view 

 maj be cited facts Buch as that Btated by Blanchard ('94) in regard to 

 Ozobranchns, that one ol the three rings of ;i typical somite is doubled 

 in the case of large individuals. 



Sr mm \hy. 



1. The number of somites in the body <>f the leech has been deter- 

 mined correctlj by Whitman ('92) for the Rhynchobdellidse, by Bristol 

 (*99) for the Gnathobdellidse. In both cases the number is thirty-four. 



2. The limits of the leech somite have been placed incorrectly by all 



Btudents <>t' leech metamerism, with the possible except! t Vaillant 



('?()) in tin- case of a .single genus, Pontobdella, from the time of 

 Gratiolet ('62) to the present. 



.;. The natural and Hue limits of the somite coincide with the limits 

 of the neuromere ; that is. :i Boniite includes those annuli which typically 

 are innervated from the same nerve ganglion. 



4. The foregoing statement is confirmed by an examination of meta- 

 merically repeated structures other than ganglia; namely. Bepta, testes, 

 and crop diverticula. 



."». Neuromeric groups of rings, that is, Bomitea a- defined under :>, 

 behave as structural units (a) in somite abbreviation (reduction in the 

 number of rings in a somite), (b) in somite elongation (increase in 

 the number of rings in a somite). 



f). Both reduction and increase in the number of rings take place 

 chiefly at the ends of the somite. The sensory ring occupies the middle 

 of the somite and is least often and least extensively affected in the two 

 processes just named. It represents the sttible component of the somite. 



7. The five-ringed type of somite found in the Gnathobdellidse has 

 been derived from the three-ringed type found in the Rhynchobdellidse, 

 a> -invested by Whitman and demonstrated by Bristol. This has been 

 brought about by division of the non-sensory ring :>' either end of 

 the somite. 



8. The wide prevalence of ring multiplication among the Birudinea 

 BUggestS the derivation of the three-ringed type of somite from a still 

 simpler type consisting, as in Chsetopoda, of :i Bingle ring. 



'.•. A phylogenetically intermediate Btage between the one-ringed and 

 three-ringed types of somite is probably represented in a typical body 

 somite of Branchiobdella. The same type of somite structure appeals 



