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a large anterior cerebral vesicle (in the wall of which an 

 eye and an otocyst (statocyst) are formed), a thickened 

 median part which becomes the trunk ganglion, and a 

 thin posterior portion in the tail. The notochord is re- 

 stricted to the tail region ( Urochorda). The body is com- 

 pletely invested by a thin cuticle, an excretion of the 

 ectoderm ; this is the tunic or test, and an outgrowth of 

 it is the delicate fin which fringes the muscular tail. 



When hatched out, the tadpole-like larva leads a free- 

 swimming life, but only for a few hours ; and it does not 

 feed, because the mouth and anus are covered in by the 

 cuticle. Then it settles down head foremost, fixing itself 

 by three papillae on the front of the head, and retrogressive 

 metamorphosis begins. The tail atrophies, is absorbed, 

 and with it the notochord and caudal nerve-cord disappear. 

 The eye and the otocyst are also lost (except Salpa, Doliolum, 

 and the Appendiculariae) ; only a part of the nervous 

 system remains, that from which tVm tmr|k ganglion, is 

 developed. The atrium enlarges, the two portions coalesce 

 dorsally, and it opens to the outside by a single atrial 

 aperture. Numerous gill-slits are formed. The mouth, 

 which was near the fixed end, becomes shunted round to 

 the opposite end, and the hind end is brought near the 

 fixed end ; there is a marked change from the horizontal 

 to the vertical in the formation of the inert and degenerate 

 adult. 



In the life-history of certain Tunicates (e.g., Molgnla, 

 Pyrosoma, Salpa) the tadpole larva is absent. 



Describe the general Appearance and Structure of an adult 



simple Ascidian (e.g., Ascidia}. 



Ascidia mentula is sac-like and roughly oval ; it is 

 greenish-grey, and its average height is about 3 inches. 

 The lower and broader end is fixed to rock or other surface ; 

 the upper or anterior end has two siphon-like openings, 

 the inhalent aperture or mouth, and neat- it (on the dorsal 

 side) the exhalent or atrial aperture. When the sphincter 

 muscles of these siphons suddenly contract, water is squirted 

 out; hence the popular name, "sea-squirts." The thick 

 opaque cuticle covering the body forms a cellulose tunic 

 or test. It is a loose tunic ; about the openings and 

 where blood-vessels enter it, there only it is attached to 

 the underlying body- wall or mantle. The mantle consists 

 f ectoderm upon a matrix of connective tissue, muscle 

 fibres, and blood spaces. 



The mouth leads into a short stomodaeum (lined with 

 cuticle) at the end of which is a sphincter muscle with a 



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