STUDIES ON THE PROTOCHORDATA. 353 



invagination of the dorsal wall of the pericardium (of. van 

 Beneden and Julin^ 7). 



In Ciona, on the contrary_, the septum dividing the two 

 halves of the pericardium is quite complete, and does not break 

 down, and the heart forms by a splitting apart of the two 

 layers composing the septum (figs. 12 and 30). Later on 

 striated muscular fibres make their appearance on the inner 

 surface of the cellular membrane which constitutes the wall 

 of the heart, being apparently the product of the cells com- 

 posing it. The dilated portions of these myo-epithelial cells 

 containing the nuclei project towards the cavity of the 

 pericardium, while the actual lumen of the heart is bounded 

 by muscle-fibres alone, with no trace of an endothelium. 



In view of the origin and structure of the heart of the 

 Tunicata, as well as on account of its remarkable and universal 

 recurrent action and the absence of a heart and pericardium in 

 Amphioxus, I am disposed to agree with van Beneden and 

 Julin that it is a special organ in the group of the Uro- 

 chorda, and is not homologous with the heart of the higher 

 Vertebrates. 



10. Summary of Principal Results. 



(1) The first four primary stigmata ofCionaintestinalis 

 are developed from one primitive gill-slit ; ii and ni are formed 

 by constriction from i and iv ; and the latter represent the two 

 halves of one slit separated by the precocious development of 

 a tongue-bar. 



(2) Three pairs of gill-slits, in the guise of six primary 

 stigmata, are represented in Ciena and other simple Ascidians. 



In Ciona the innumerable branchial stigmata of the adult 

 are derived by subdivision from these six primary stigmata, 

 and not by new perforations. 



(3) The endostyle of Ciona is at first quite anterior in posi- 

 tion, lies in a dorso-ventral direction in front of the mouth, 

 and its primary long axis is at right angles to its definitive 

 long axis. 



(4) The cavity in the fixing stolon is the praeoral or anterior 



