430 



TUNICATA. 



of eye is a mound-like swelling of the brain (Fig. 215 A) ; at the sides of this 

 swelling are arranged the pigment-ce^ls while, from the centre, closely packed 

 rod-bearing cells are found radiating 

 towards the surface, the nerve-fibres 

 being connected with the inner ends of 

 the rods. In this eye, the rods are 

 therefore turned directly towards the 

 source of light. In other forms, the 

 rudiment of the eye becomes differenti- 

 'ated into three parts which either remain 

 united in the shape of a horse-shoe, or 

 form three entirely distinct eyes, the 

 unpaired median eye retaining the 

 original simple condition (Fig. 216 B, a) 

 while the two lateral eyes are formed 

 on the plan of an inverse eye (b), i.e., the 

 rods are directed away from the surface 

 and the nerve -fibres are connected with 

 their outer ends. BUTSCHLI, assuming 

 an optic vesicle which cannot be observed, 

 homologised the median non-inverted 

 eye with the cephalic eye of the Verte- 

 brates, and the lateral inverted eyes with 

 the paired eyes of the Vertebrates, but r 

 independent of this theoretical vesicle, 

 the structure of the lateral eye of the 

 Ascidian larva seems directly to suggest the paired vertebrate eyes by the 

 fact that the rods are directed towards the cerebral cavity (see, however, the 

 objections raised by METCALF, No. 99, and GOPPERT, No. 94a). 



The first rudiment of the pharynx and the development of the gill 

 have already been described (p. 426). The wall of the pharyngeal 

 cavity is formed by a simple epithelium, the cells are either cubical 

 or somewhat flattened. The rudiment of the endostyle (hypobranchial 

 furrow) appears in the form of paired folds of this epithelium origin- 

 ating at some distance from each other (Fig. 216, es) ; these shift 

 towards one another later and then form the boundaries of a hypo- 

 branchial furrow which runs from the peripharyngeal bands to near 

 the entrance of the oesophagus. The rudiments of the peripharyngeal 

 bands which run towards the anterior end of the gill from the anterior 

 end of the endostyle, encircling the aperture of the respiratory cavity, 

 first appear as similar prominences. The rudiments of the branchial 

 and atrial apertures (Fig. 216, i and e) appear only in later stages 

 in the form of transverse depressions of the ectoderm which break 

 through into the pharynx and atrial cavities. The rudiment of the 

 atrial aperture originally lies almost at the centre of the dorsal side 



FIG. 215. Diagram of the eye oi' a 

 Salp (after BtflSCHU). A, typical 

 single eye ; B, typical tripartite eye. 

 a, median part, b, lateral part of 

 the tripartite eye ; n, nerve-fibres ; 

 p, pigment-cells ; r, retina. 



