UROCIIORDA. 21 



tinuous round the branchial sack, except along the ventral line 

 where the endostyle is present. The atrial cavity, from its 

 mode of origin as a pair of epiblastic involutions 1 , is clearly a 

 structure of the same nature as the branchial or atrial cavity of 

 Amphioxus; and has nothing whatever to do with the true body 

 cavity. 



It has already been stated that the anus opens into the 

 original left atrial cavity; when the two cavities coalesce the 

 anus opens into the atrial cavity in the median dorsal line. 



Two of the most obscure points in the development are the 

 origin of the mesoblast in the trunk, and of the body cavity. 

 Of the former subject we know next to nothing, though it seems 

 that the cells resulting from the atrophy of the tail are em- 

 ployed in the nutrition of the mesoblastic structures of the 

 trunk. 



The body cavity in the adult is well developed in the region 

 of the intestine, where it forms a wide cavity lined by an 

 epithelioid mesoblastic layer. In the region of the branchial 

 sack it is reduced to the vascular channels in the walls of the 

 sack. 



Kowalevsky believes the body cavity to be the original seg- 

 mentation cavity, but this view can hardly be regarded as 

 admissible in the present state of our knowledge. In some 

 other Ascidian types a few more facts about the mesoblast will 

 be alluded to. 



With the above changes the retrogressive metamorphosis 

 is completed ; and it only remains to notice the change in 

 position undergone in the attainment of the adult state. The 

 region by which the larva is attached grows into a long process 

 (fig. 10 B), and at the same time the part carrying the mouth is 

 bent upwards so as to be removed nearly as far as possible from 

 the point of attachment. By this means the condition in the 



1 In the asexually produced buds of Ascidians the atrial cavity appears, with the 

 exception of the external opening, to be formed from the primitive branchial sack. 

 In the buds of Pyrosoma however it arises independently. These peculiarities in the 

 buds cannot weigh against the embryonic evidence that the atrial cavity arises from 

 involutions of the epiblast, and they may perhaps be partially explained by the fact 

 that in the formation of the visceral clefts outgrowths of the branchial sack meet the 

 atrial involutions. 



