464 ALFRED GIBBS BOURNE. 



(? number, about 10) of small dilatations which exhibit rhythmical 

 contractions, and which are homologous with similar organs in 

 Piscicola and Pontobdella, and I have little doubt with 

 the dilatations at the base of the branchiae in Branchellion. 

 As I have shown in Pontobdella, these dilatations form a 

 medium of communication between the two blood- systems, 

 thus rendering it highly probably that there is a similar com- 

 munication in the case of Clcpsine. 



• Pisci-cola and Branchellion. 



The slight extent to which I have been able to carry my 

 observations in Piscicola and Branchellion forbids my 

 criticising the existing observations of Leydig and De Quatre- 

 fages, or giving any full account of the circulation ; I may, 

 however, record a few isolated observations which seem to 

 connect these genera with Clepsine and Pontobdella in 

 this respect. 



Piscicola. — As Leydig has pointed out the dorsal vessel 

 lies in a dorsal sinus. 



In its anterior portion it gives off several branches which 

 form the ventral vessel as in Clepsine. 



I regard it as probable that Leydig is mistaken here, as in 

 the case of Clepsine, with regard to the dorsal vessel being 

 open posteriorly ; and that, as in that genus, it runs on and 

 communicates posteriorly also with the ventral vessel. 



Branches similar to those described above in Pontob- 

 della leave the lateral vessel and pass into the skin opening 

 into a contractile dilatation, returning thence to communicate 

 with the sinous system; but this latter is mere conjecture. 



Branchellion. 

 Moquin Tandon states that the " dorsal vessel " is double. 

 This De Quatrefages denies, but Moquin-Tandon was right. 

 The dorsal vessel generally lies in the dorsal sinus, but comes 

 outside it more often than in Pontobdella. De Quatrefages 

 states that the ventral vessel is double. The ventral vessel 

 lies for the greater part of its length outside the ventral sinus. 



