14 THE TEST. 



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 archenteric 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 invest- 

 ment 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 papillae, deve- 

 loped 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 papillae have become formed cells first make their appear- 

 ance 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 con- 

 siderable length the egg-membrane bursts, and the larva becomes 

 free. The hatching takes place in Asc. canina about 48 60 

 hours after impregnation. The free larva (fig. 8 V.) has a 

 swollen trunk, and a very long tail, which soon becomes 



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. 



z It is probable that these papillae are very primitive organs of the Chordata. 

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

 the larva^ of Amphibia, and in front of the mouth in the larvce of Ganoids (Acipenser, 

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



