ON THE PERIVISOEEAL CAVITY OF CIONA. 127 



Appendicularia. He considers that the further development 

 of the atrial cavity has reduced and obliterated this cavity in 

 the other simple Ascidians. This view is not supported by 

 the above facts, which lead us to look upon the perivisceral 

 cavity of Ciona as a specially modified epicardium which has 

 become greatly enlarged. The perivisceral cavity is certainly 

 not homologous with the general blastocoele space of Appen- 

 dicularia, and we have no reasons for believing that the other 

 simple Ascidians pass in development through a stage in which 

 the epicardium is modified and enlarged, as in Ciona, to be 

 afterwards reduced ; so that we cannot look upon the perivis- 

 ceral cavity of Ciona as a primitive condition : in this respect 

 Ciona is the most modified of the simple Ascidians. 



In conclusion, it is my pleasant duty to express my sincere 

 thanks to Mr. Harmer for his kind help and assistance, to the 

 various members of the stafi" of the Zoological Station at 

 Naples for the great kindness I received at their hands, and 

 to the Master and Fellows of Christ's College for the grant of 

 a scholarship during my residence at Naples. 



References. 



1. L. RouLE.— " Recherches sur les Ascidies simples des cotes de Provence," 



' Annales du Musee d'Histoire Naturelle de Marseille,' vol. ii, 1884-5. 



2. E. VAN Beneden and C. Julin.—" Recherches sur la morphologic des 



Tuniciers," 'Archives de Biologic,' vol, vi, 1887. 



3. W. A. Herdman. — " On the Evolution of the Blood-vessels of the Test in 



the Tunicata," ' Nature,' vol. xxxi, 1885, p. 247. 



4. W. A. Hebdman. — "Report on the Tunicata," part iii, 'Challenger 



Reports,' " Zoology," vol. xxvii. 



