CASTLE. — METAMERISM OF THE HIRUDINEA. 



297 



1 . RhynchohdellidcE. 



We are already sufficiently familiar with the typical three- ringed 

 somite of Glossiphonia (Fig. 6, C). In Hsementeria, a closely related 

 genus (Fig. 6, D), each of the non-sensory rings (1 and 3) is divided 

 ventrally but not dorsally. 



Branchellion has a three-ringed somite like that of Glossiphonia, ex- 

 cept that the middle (sensory) ring bears a pulsating respiratory vesicle 



D 



B 



m-- 



Fig. 6. Somites of Rhynchobdellidse. 



A, a much abbreviated somite of Glossiphonia ; 



B, a less abbreviated somite of Glossiplionia ; 



C, a typical somite of Glossiphonia; 



D, " " flEementeria ; 



E, " " Pontobdella; 



F, " " Trachelobdella; 



G, " " Cystobranchus ; 

 The sensory ring and its derivatives are stippled. 



H 



I 



K 



M 



1 



2 



3 



4 



5 



'■--, 



3J 



Fig. 7. Typical somites of Gnathobdellidae. E, Nephelis ; /, Dina ; J-il, 

 Trocheta. Tlie sensory ring and its derivatives are stippled. 



connected with the lymph system. In Ozobranchus, a related genus, 

 one of the non-sensory rings is divided in large individuals. Blanchard 

 ('94), who has described this genus, states that the ring which divides is 

 the third ring of the somite, by which I understand him to mean the ring 

 yi\nc\\ precedes the sensory ring, — ring 1 of my enumeration. 



In Pontobdella (Fig. 6, E), a typical somite consists sometimes of 



