ON THE PEEIVISOEEAL CAVITY OF CIONA. 125 



middle of metamorphosis, while the larva is still fixed by a 

 long stalk ; it shows the formation of the heart by the invagi- 

 nation of the dorsal wall of the pericardium just as it has been 

 shown to be formed in Clavellina. 



Fig. 6 is a transverse section through a larva at this stage, 

 passing through the heart (h.), which is seen to be formed by 

 the invagination of the dorsal wall of the pericardium (pc), 

 and to be still open to the general blastocoele space (b.), which 

 is now very much reduced, the chief portion of it being con- 

 fined to a small space round the alimentary canal, into which 

 the heart opens in the neighbourhood of the stomach {st.). 

 The whole of the viscera are now surrounded by a large peri- 

 visceral space [ep.), which may be distinguished from the 

 general blastocoele space by its containing no free cells. This 

 space evidently corresponds to the epicardiac tubes of the 

 preceding stage, now very much enlarged and fused together 

 completely, except at the region of the pericardium between 

 the pharynx and the stomach. A comparison of this stage 

 with the preceding, and with the development of Clavellina as 

 described by van Beneden and Julin, can leave no doubt that 

 the pericardium in Ciena is separated off" from the epicardiac 

 tubes just as it is in Clavellina, and also that the perivisceral 

 cavity of Ciona corresponds to the epicardium of Ciona. 



The later stages of development, as regards the perivisceral 

 cavity and associated organs, are very simple ; the heart 

 becomes completely closed except at each end, where it still 

 opens to the original blastocoele space, which becomes reduced 

 to the blood-vessels ; the two halves of the perivisceral cavity 

 approach each other between the pericardium and stomach till, 

 on the closure of the heart and the reduction of the blood- 

 space round the stomach, they only become separated by their 

 thin walls, which unite to form the mesentery attaching the 

 pericardium to the stomach, a similar process also taking place 

 on the ventral side of the pericardium between it and the 

 pharynx. 



The derivation of the heart from the dorsal wall of the 

 pericardium is still indicated in the adult, where the heart, 



