WITH NOTES ON THE WEST INDIAN SPECIES. 



319 



It is a subject of great complexity, and I can only indicate some of the facts and 

 arguments which must be brought to bear upon it. 



1. In virtue of their medullary and peribranchial properties, the pleural folds 

 which are represented in the Enteropneusta by the genital pleurae must contain 

 within them the primordia not only of medullary folds, but also of atrial folds. 



2. Although Amphioxus is not the only animal which possesses an atrium, it is 

 the only animal in which the atrium is formed by the fusion of atrial (metapleural) folds. 



3. Amphioxus possesses atrial folds, but not medullary folds \ the central nervous 

 system forming cenogenetically by delamination. 



4. The two halves of the Tunicate atrium are confluent dorsally. 



5. The two halves of the atrium of Amphioxus are confluent ventrally. 



6. The atriojDore of Amphioxus is a neoformation. It is neither an orifice of 

 invagination nor does it arise ontogenetically as a perforation of the body-wall, but 

 it is a foramen remaining after fusion of folds. 



7. Several species of the subgenus Tauroglossus are characterised by the presence 

 of deep ventral coecal prolongations of the gill-pouches {Pt carnosa, etc.). 



8. In describing the condition met with in Amphioxus in terms derived from 

 the comparison of Amphioxus with a form like Pt. carnosa, we should say that in 

 Amphioxus the dorsal gill-pores are lost, the gill-pouches of each side are confluent 

 longitudinally, and the gill-pouches of both sides are confluent ventrally, while the 

 atriopore is a neoformation. 



9. In order to appreciate the condition met with in a form like Pt. carnosa as 

 compared with Pt. flava, a glance at PL XXX. Fig. 22, will show that if the septal 

 walls dividing the successive gill-pouches from one another were to break down, so 

 that the gill-pouches of each side became confluent longitudinally, we should have 

 absolutely the condition which we actually find in Pt. flava (PI. XXVIII. Fig. 6). 



10. The ventral coeca of the gill-pouches of Pt. carnosa and other species are 

 reminiscent of the ventral origin of the genital pleurae (compare PI. XXX. Fig. 22, 

 and PI. XXVIII. Fig. 6). 



11. Apart from the implication contained in the preceding hypothesis (No. 10), 

 there is every reason to regard Pt. fl.ava as a relatively primitive form. 



12. Hence the gill-pouches of the Enteropneusta are structures superadded to the 

 primitive organisation. 



The broad generalisation which may be formulated as the summation of the 

 preceding considerations is that the genital folds of Enteropneusta, the atrial folds of 

 Amphioxus, and the medullary folds of Vertebrata belong to the system of pleural folds 

 of the body-wall, and are differentiations from a common primordium. 



' An interesting example of compensating growth. 



W. III. 



44 



