CHAPTER 15 



II. UROCHORDA 



A great deal of interest has been shown in the study of the " single " 

 eye which is found in the ventricular cavity of the cerebral vesicle of the 

 free-swimming larva of A scidia mammillata or Phallusia (Figs. 3, Chap. 1, 

 p. 5, and Fig. 127). Besides the eye, a single statocyst is present in the 

 expanded anterior end of the neural tube, which expansion is usually spoken 

 of as the " sense vesicle." In Phallusia the anterior neuropore remains open 

 for a certain period during the larval stage. Later, regressive changes set in 

 which precede the fixed, plant-like life of the adult animal, and the vesicle 

 with its contained sensory organs disappears, the central nervous system 

 becoming reduced to a small ganglion with associated nerve-fibres. 

 The so-called single eye, according to Froriep, corresponds to the right 

 lateral eye of vertebrates, and he has shown that there is a rudiment of a left 

 eye also present, which is connected by nerve fibres with the neural tube. 

 These fibres join the central nervous system at a point corresponding 

 to the termination in the neural tube of the optic nerve of the right 

 eye. The mode of development of this right lateral eye is indicated 

 in Fig. 127, which shows an enlargement and elongation of certain cells 

 forming a part of the right wall of the cerebral vesicle and a deposit 

 of pigment on their inner or ventricular aspect. This placodal thickening 

 of the wall of the cerebral vesicle is later converted into an optic vesicle, 

 which instead of projecting outwards towards the cutaneous ectoderm 

 becomes enclosed in the ventricular cavity. The " lens " and " retina " 

 are formed by the clear cells which originally lay to the outer side of the 

 pigment-mass and formed part of the wall of the cerebral vesicle. The 

 lens is thus formed from neural ectoderm, not cutaneous ectoderm as in 

 the lateral eyes of vertebrates generally, and there is no invagination of 

 the outer wall of the optic vesicle to form an optic cup. 1 Light can only 

 reach the eye-vesicle through the transparent body-wall and the wall of 

 the cerebral- or sense-vesicle. 



Owing to the eye of the ascidian larva being single, Salensky and 

 others considered that it could not be homologous with the paired lateral 



1 It may be presumed that the absence of a lens developed from the cutaneous 

 ectoderm is probably due to absence of the invagination of the neural ectoderm 

 to form an optic cup. (R. J. G.) 



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