2O 



METAMORPHOSIS. 



BRAIN 



The openings of the atrial cavity at first are shut off from 

 communication with the exterior by the test, but not long after 

 the larva becomes fixed, two perforations are formed in the test, 

 which lead into the openings of the two atrial cavities. At the 

 same time the atrial cavities dilate so as gradually to embrace the 

 whole branchial sack to which their inner walls attach themselves. 

 Shortly after this the branchial clefts rapidly increase in 

 number 1 . 



The increase of the branchial clefts is somewhat complicated. Between 

 the two primitive clefts two new ones appear, and then a third appears 

 behind the last cleft. In the interval 

 between each branchial cleft is placed 

 a vascular branchial vessel (fig. 8 vi. 

 bb\ Soon a great number of clefts 

 become added in a row on each side 

 of the branchial sack. These clefts 

 are small ciliated openings placed 

 transversely with reference to the 

 long axis of the branchial sack, but 

 only occupying a small part of the 

 breadth of each side. The intervals 

 dorsal and ventral to them are soon 

 filled by series of fresh rows of slits, 

 separated from each other by longi- 

 tudinal bars. Each side of the 

 branchial sack becomes in this way 

 perforated by a number of small 

 openings arranged in rows, and 

 separated by transverse and longitu- 

 dinal bars. The whole structure forms the commencement of the branchial 

 basketwork of the adult ; the arrangement of which differs considerably in 

 structure and origin from the simple system of branchial clefts of normal 

 vertebrate types. At the junction of the transverse and longitudinal bars 

 papillas are formed projecting into the lumen of the branchial sack. 



After the above changes are far advanced towards com- 

 pletion, the openings of the two atrial sacks gradually approxi- 

 mate in the dorsal line, and finally coalesce to form the single 

 atrial opening of the adult. The two atrial cavities at the same 

 time coalesce dorsally to form a single cavity, which is con- 



TAIL- 



FIG. IT. DIAGRAM OF A VERY YOUNG 

 ASCIDIAN. (From Lankester.) 



1 The account of the multiplication of the branchial clefts is taken from Krohn's 

 paper on Phallusia mammillata (No. 24), but there is every reason to think that it 

 holds true in the main for simple Ascidians. 



