CASTLE. 



METAMERISM OP THE HIRUDINEA. 



297 



1 . Rhynchobdellidce. 



We are already sufficiently familiar with the typical three-ringed 

 somite of Glossiphonia (Fig. G, C). In Haementeria, a closely related 

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

 ventrally hut 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 



A 



B 



m- 



m 



j... 



m 



Fig. G. Somites of Rhynchobdellidae. 



A, a much abbreviated somite of Glossiphonia; 



B, a less abbreviated somite of Glossiphonia ; 



C, a typical somite of Glossiphonia; 

 I>, Haementeria; 



E, " " Pontobdella; 



F, " " Trachelobdella; 



G, Cystobranchus ; 

 The sensory ring and its derivatives are stippled. 



H 



I 



K 



M 



1 



2 



3 



4 



5 



5: 



-7i 



Fig. 7. Typical somites of Gnathobdellidse. B, Nephelis ; /, Dina ; J-M, 

 Trocheta. The 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. Blauchard 

 ('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 

 which precedes the sensory ring, — ring 1 of my enumeration. 



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



