406 



UROCHORDA 



[CH. 



but the spot where this occurs is involved in the invagination which 

 forms the stomodaeum. The tube connecting the sense-vesicle and 

 the stomodaeum is called the neuropore (2, Fig. 199). Part 



of the side- wall of this 

 vesicle is modified so as 

 to form a cup-shaped eye 

 with a simple cuticular lens 

 directed inwards. From the 

 floor rises a pillar of cells 

 on which a ball of calcar- 

 eous matter' is balanced ; 

 this acts as an otolith, 

 and the whole forms a rudi- 

 mentary ear. 



Thus both in the struc- 

 ture of the nervous system 

 and the position of the 

 anus, the tadpole of Tuni- 

 cata is more advanced than 

 the AmpMoxus. 



Although, as we have 

 seen, both mouth and anus 

 are present, yet they cannot 

 be used, for they are closed 

 by a sheet of gelatinous 

 matter. This is the test 

 which is secreted by the 

 ectoderm cells and en- 

 velops the whole body, 

 so that during its brief 

 free-swimming life the As- 

 cidian takes no food. 



After swimming for aj 

 short time the larva fixes 

 itself by its 

 chin-warts to a 

 suitable sub- 

 stratum and undergoes a 



very rapid metamorphosis. The tail shrinks and is absorbed, 

 notochord and nerve-tube disappear : the sense-vesicle also dis- 

 appears, only its hinder thickened wall persisting as the adult 



6, 



FIG. 201. Diagram of the fixing and changes 

 undergone by a larval Ascidian. From 

 Lankester. 



1. Mouth. 2. Anus. 3; Gill-slits. 

 4. In A, notochord; in B and C, vanishing 

 tail. 5. In A, tail. 6. Brain. 



Metamor- 

 phosis. 



