ANATOMY AND DEVELOPMENT. 203 



sole organ of locomotion, and hence of transportation from 

 place to place ; this only being possible during the larval 

 period. 



As a rule, the larval phase of an Ascidian's existence is 

 a remarkably brief one, and there is on this account all the 

 more need for an effective propelling organ, which will 

 enable the larva to arrive at a suitable resting-place. 



In Amphioxus, as described above, locomotion is ef- 

 fected by serpentine movements of the whole trunk in 

 virtue of its muscle-segments, and there is therefore no 

 need for a tail in addition ; but there is, nevertheless, a 

 short post-anal extension of the body, which alone can be 

 regarded as the homologue of the tail of the Ascidian larva. 

 In the latter (e.g. Ciona, Phallusia, etc.) the muscles are 

 entirely confined to the tail, none being formed in the body 

 proper, until after the resorption of its caudal appendage. 



On the view which I am endeavouring to make clear, 

 it follows that the tail of the Tunicate tadpole is of the 

 same nature as that of the Amphibian tadpole, and, in fact, 

 of the craniate Vertebrates generally, and, as has just been 

 said, is only represented by the short post-anal section of 

 the trunk in Amphioxus. 



The solid cord of endoderm in the tail is not, therefore, 

 a rudiment of a primitive intestine, but it is analogous to, 

 even if not, as first suggested by BALFOUR, homologous 

 with, the so-called post-anal gut which occurs in the em- 

 bryos of the higher Vertebrates, and bears a similar rela- 

 tion to the formation of the tail that the endoderm-cord in 

 the Ascidian embryo does. 



Thus in the typical Ascidian embryo the elongation of 

 the trunk (body proper) does not take place to any consid- 

 erable extent during the embryonic or even larval period, 

 but only after the metamorphosis. 



