TTPE PROTOCHORDATA. 625 



are, however, to be noticed ; the atrial cavity (at) is yet 

 quite small and the anus (a) may or may not communicate 

 with it, while the mouth (in) is not functional, owing to the 

 test being continuous over it. The nervous system is the 

 most remarkable structure, however, consisting of a large an- 

 terior saclike structure, the brain (ce), into the cavity of which 

 two sense-organs project. Upon the dorsal wall is the eye, 

 consisting of a cup-shaped group of cells whose inner ends 

 are pigmented, the hollow of the cup being occupied by a 

 lens above which is a membrane, the so-called cornea. Pro- 

 jecting dorsally from the ventral wall is the otolith, consist- 

 ing of a pear-shaped cell bearing a cap of pigment, its smaller 

 end extending between the cells of the lower wall of the 

 brain, which in its neighborhood are provided with stiff cells 

 forming a crista acustica. 



Posteriorly the brain is prolonged into a thick-walled tube 

 (ri) which extends back almost to the tip of the tail, to the 

 muscles of which it sends off nerves. Beneath the nerve- 

 tube, throughout nearly its entire length, lies a notochord (nc) 

 which serves as a skeletal support to the tail, and on each 

 side of it is a plate of longitudinal muscle-fibres. 



By means of energetic lateral movements of the tail this 

 larva swims about for some time, but when about to trans- 

 form itself into the adult it fastens to some solid object by 

 one of the adhesive papillae. The tail with its nervous sys- 

 tem, muscles, and notochord then undergoes degeneration 

 and is completely absorbed, the portion of the test covering 

 it being thrown off, and a rotation of the body takes place so 

 that the mouth comes to lie at the opposite end of the body 

 from the point of fixation. The branchial and atrial aper- 

 tures form, and the anterior saclike brain collapses, the 

 sense-organs degenerate, and the adult brain is gradually 

 developed. The extension of the atrium and the develop- 

 ment of additional stigmata complete the acquisition of the 

 adult characteristics. 



It will be seen from this, then, that the larvae are of great 

 importance in estimating the systematic position and the 

 affinities of the Urochorda, and that the adults, except in the 

 Appendicularice, in which the tail and the free-swimming habit 



