ANATOMY AND PHVSIOLOGV OF SALPA AND PYROSOMA 55 



Ascidian " zooids " \ying perpendicular to the axis of the tube, and 

 united together by a common cartilaginous basis; and the small 

 circular apertures correspond respectively, the outer to the anterior 

 aperture of the Salpa, the inner to the posterior aperture. 



Each aperture is provided with a small dentated membranous valve, 

 Plate XVII. [Plate 7] fig. i. 



45. In eachzooid there is at one point a ganglion {d), with a mass 

 of deep red otolithes. As in Salpa, this must be called the ventral 

 side ; the opposite is the dorsal side, and contains (as in Salpa) an 

 endostyle, Plate XVII. [Plate 7] figs, i, 2 r. The ganglionic or ventral 

 surfaces of all the polypes are turned the same way, and towards the 

 open end of the cylinder. 



By far the greater part of the space occupied by each zooid is taken 

 up by the respiratory cavity. This is elliptical, and compressed 

 laterally. It is lined by the proper branchial network, hereafter to be 

 described {z>), and communicates freely by means of the apertures in 

 the branchial network with the post-branchial or anal cavity, which, 

 as before stated, opens into the interior of the cylinder. 



46. The viscera lie behind the branchiae. They consist of the 

 digestive canal, heart, and generative organs. 



The intestine (r, s, i) commences by a wide mouth with thick lips, 

 at the posterior, ventral extremity of the respiratory cavity. The 

 oesophagus (r) runs back, and then upwards to terminate in the wide 

 subquadrilateral stomach {f). A narrow pylorus communicates with 

 the intestine, which passes at first upwards and forwards, and then 

 suddenly becoming bent upon itself, runs downwards and to the right 

 side, to end in the wide flattened anus (r). 



The oesophagus is dotted over with branched carmine pigment- 

 cells; and similar cells are frequently seen upon the intestine just 

 beyond the pylorus. 



47. A tubular axis [u) arises from the stomach, and branches out 

 on the rectum into a system of tubes as in Salpa ; but the ramifica- 

 tions are less numerous and less regular, with wider meshes than in 

 the latter. The tubes are less transparent and have more the appear- 

 ance of solid fibres, and finally they terminate towards the anus in 

 wide globular caeca. 



The stem of this system is about yTjVirth of an inch in diameter. 



48. Each zooid is composed of two tunics, an outer (a), confluent 

 with the general cartilaginous basis, and an inner (/3) continuous 

 with the outer at its anterior and posterior extremities, and adherent 

 to it antero-laterally, in two oval spots, one on each side, which, when 

 examined by the microscope, appeared to consist of nothing more 



