STUDIES ON THE PEOTOCHORDATA.. 347 



As shown b)'^ Harmer (17) the resemblance between the 

 organisation of Cephalodiscus and Balanoglossus is an 

 astonishingly close one, the main essential difference being in 

 the shape of the alimentary canal in the two forms, but it has 

 not yet been suggested that the intestine of Cephalodiscus is 

 not homologous with that of Balanoglossus. In the familiar 

 instance of Phoronis we have a straight intestine in the larva 

 and a V-shaped one in the adult, the transition from the one 

 condition to the other being readily observable. 



The relationship of the Ascidians to Amphioxus seems to 

 me to be perfectly analogous to that between Cephalodiscus 

 and Balanoglossus, as the tabular scheme given below will 

 show. 



The homology of the intestine of the Ascidians with that of 

 Amphioxus having now been established, at least on a basis of 

 probability, it is interesting to note that there is also a csecal 

 diverticulum of the former which would correspond with the 

 so-called hepatic csecum of the latter. The origin of this 

 caecum was first described by Krohn (23) in the case of Phal- 

 lusia mammillata, but without figures, and he made the 

 singular mistake of supposing that it was at first solid. He 

 says (p. 331), " Sehr friih . . . zeigt sich am Anfange des 

 Darmes gleich hinter dem Magen ein cylindrischer, gegen sein 

 freies Ende hin etwas keulenformig verdickter, durchweg 

 homogener Fortsatz [later on he uses the term solid] der 

 quer zur linken Seite bis in die Nahe der hier gelegenen End- 

 portion des Darmes sich erstrecht." It then begins to branch 

 dichotomously, and becomes the pyloric gland of the adult. 



In Ciona, as a matter of fact, this csecum arises as a simple 

 blind hollow outgrowth from the stomach, just at the point 

 where the intestine leaves the latter. It probably commences 

 to grow out at the stage represented in fig. I, but at any rate 

 is clearly visible in the succeeding stages (figs. 2 — 5). Fig. 

 19 is a ventral view showing the csecum growing out as a 

 diverticulum of the stomach, with a distinct lumen lined by a 

 columnar epithelium. In the next stage (figs. 3 and 4) the 

 csecum, M-hich, as stated by Krohn, follows the course of the 



