12 THE TEST. 



During the above changes the tail becomes considerably elongated 

 and, owing to the larva being still in the egg-shell, is bent over to 

 the ventral side of the trunk. 



The larva at this stage is represented in a side view in fig. 8 IV. 

 The epidermis is formed throughout of a single layer of cells. In 

 the trunk the mesenteron is shewn at dd and the dilated part of the 

 nervous system, no longer communicating with the exterior, at n. 

 In the tail the notochord is shewn at ch, the muscles at m, and the 

 solid remnant of the ventral wall of the archenteron at dd'. The 

 delicate continuation of the neural canal in the tail is seen above the 

 notochord at n. An optical section of the tail is shewn in fig. 6. 

 It is worthy of notice that the notochord and muscles are formed 

 in the same manner as in Amphioxus, except that the process 

 is somewhat simplified. The mode of disappearance of the archen- 

 teric cavity in the tail, by the employment of the whole of its walls 

 in the formation of various organs, is so peculiar, that I feel some 

 hesitation in accepting Kowalevsky's statements on this head 1 . 



The larva continues to grow in length, and the tail becomes 

 further curled round the ventral side of the body within the egg- 

 membrane. Before the tail has nearly reached its full length the test 

 becomes formed as a cuticular deposit of the epiblast cells (O. Hertwig, 

 No. 13, Semper, No. 37). It appears first in the tail and gradually 

 extends till it forms a complete investment round both tail and 

 trunk, and is at first totally devoid of cells. Shortly after the 

 establishment of the test there grow out from the anterior end of the 

 body three peculiar papillui, developed as simple thickenings of the 

 epidermis. At a later stage, after the hatching of the larva, these 

 papillae develop glands at their extremities, secreting a kind of 

 glutinous fluid' 2 . After these papilla have become formed cells first 

 make their appearance in the test ; and there is simultaneously 

 formed a fresh inner cuticular layer of the test, to which at first the 

 cells are confined, though subsequently they are found in the outer 

 layer also. On the appearance of cells in the test the latter must be 

 regarded as a form, though a very abnormal one, of connective tissue. 

 When the tail of the larva has reached a very considerable length 

 the egg-membrane bursts, and the larva becomes free. The hatching 

 takes place in Asc. cauina about 48 GO hours after impregnation. 

 The free larva (fig. 8 V.) has a swollen trunk, and a very long tail, 

 which soon becomes straightened out. It has a striking resemblance 

 to a tadpole (vide fig. 10). 



In the free larval condition the Ascidians have in many respects 

 a higher organization than in the adult state. It is accordingly 



1 It is more probable that this part of the alimentary tract is equivalent to the 

 post-anal gut of many Vertebrata, which is at first a complete tube, but disappears 

 later by the simple absorption of the walls. 



2 It is probable that these papillae are very primitive organs of the Cliordata. 

 Structures, which are probably of the same nature, are fmnied behind the mouth in 

 the larvaa of Amphibia, and in front of the month in the larva? of Ganoids (Acipenser, 

 Lepidosteus), and are used by these larvae for attaching themselves. 



