UROCHORDA. 



443 



spots, probably visual in function, are found, one between each of the lobes 

 surrounding the oral aperture in many Ascidiidae. Pyrosoma and Salpa 

 have a pigmented outgrowth of the ganglion, with a refracting body 

 imbedded. And the larva has an eye with a concave pigmented retina and 

 complex lens within the anterior dilatation of the nervous system. An 

 otolith borne on a stalk is found in the same position in the larva. The 

 Larvacea have a large vesicular otocyst containing an otolith on the left 

 side of the anterior ganglion, and the ' nurse ' in Doliolum has a similar 

 structure on the left side of the body, connected by a nerve to the 

 ganglion. 



The oral or ' inhalent ' aperture leads into a pharynx. A groove the 

 endostyle extends along the ventral aspect of this cavity, except in 

 Kowalewskaia, which has in its place a right and left row of ciliated 

 tooth-like processes. This endostyle is lined by mucus-producing cells and 

 ciliated cells. Anteriorly its lips are connected with the posterior of two 

 ciliated peripharyngeal bands surrounding the oral aperture, which is con- 

 tinuous on the dorsal aspect of the pharynx with a ciliated ridge or fold, 

 the dorsal lamina. This lamina extends backwards to the opening of the 

 'oesophagus.' Its edge is sometimes denticulated, forming a single or 

 double row of languettes. Posteriorly the endostyle is connected to the 

 oesophagus by a ciliated line, which comes also into connection with the 

 posterior end of the dorsal lamina. The mucus secreted is chiefly carried 

 forwards, and as it passes upwards to the dorsal lamina, it is drawn out 

 into long strings by the current of water entering the oral aperture, and 

 catches up all floating food-particles with which it is laden. It is then 

 conveyed along the dorsal lamina into the oesophagus. In Larvacea the 

 pharynx has two latero-ventral outlets or stigmata, one on the right, the 

 other on the left, surrounded by a ring of ciliated cells, and developed each 

 from a pharyngeal outgrowth meeting an ectodermal ingrowth. Other 

 Urochorda have a single atrial, exhalent or cloacal aperture. This aperture 

 is placed dorsally, either near the oral aperture (Ascidiae Simplices and 

 some Compositae] or at some distance from it (other Compositae\ or else at 

 the opposite pole of the body (A. Salpaeformes, Thaliacea}. It leads into a 

 ' peripharyngeal ' or atrial cavity, which surrounds the pharynx, except 

 along the line of the endostyle, and is limited anteriorly by the line 

 of the peripharyngeal bands. The cavity is lined by an epithelium 

 derived from the ectoderm, from which it is formed by one or two in- 

 vaginations. In the latter case the two apertures fuse. In certain buds, 

 however, it appears to be formed by pharyngeal outgrowths. The lateral 

 walls of the pharynx are often folded longitudinally and are perforated by 

 ciliated slits (stigmata), between which run blood-channels. The slits lead 

 into the atrial cavity. Among Thaliacea^ the pharynx in Doliolum is 

 represented by a vertical membrane perforated by only two rows of slits, 



