130 ZOOLOG 7. 



the ends of the branches, filling the body, whence it is taken 

 up by the madreporic body and carried into the water- 

 vascular system by the narrow duct on the left side of the 

 pharynx. Besides being respiratory, this organ is supposed 

 to be depuratory in its function. In some Holothnrians 

 certain organs (the Cuvierian organs), supposed by Semper 

 to be organs of defence, as they are readily thrown out when 

 the animal is disturbed, are attached either to the stem of 

 the respiratory tree or to the cloaca. The madreporic body 

 (ni) forms a rosette, partly surrounding the membrane at- 

 tached to one side of the pyloric end of the stomach, and 

 leads by the madreporic canal, which is closely^ bound down 

 to the pharynx, to the ring-canal (vr). Also connected 

 with the ring-canal are two enormous Polian vesicles (p, p), 

 which are nearly two thirds as long as the body ; by slitting 

 up their base with scissors they can be followed to the ring- 

 canal. The latter (vr) is a capacious canal surrounding the 

 mouth, and can be detected by laying open the oral-opening, 

 and then by cutting across the longitudinal muscles (as aiv) 

 the radial vessels may be followed along the body under the 

 muscles. Just above the ring-canal is situated the nervous 

 ring (nr), and its radial nerves (n) can be traced along and 

 outside of the radial water-vascular canals. The ampullae 

 (am) are red, conical, flask-shaped, conspicuous organs, lying 

 irregularly, a row on each side of each longitudinal muscle. 

 They are filled with water from the small lateral vessels of 

 the radial water-vascular canals, The single ovary is com- 

 posed of a large mass of long tubes, which are larger than 

 and tangled up with the branches of the respiratory tree. 

 The oviduct is attached by a membrane to the stomach, and 

 opens between two of the tentacles on the edge of the 

 mouth. 



The blood or pseudo-haemal vessels * are difficult, without 

 very fine dissections, to be made out. The system consists 

 of a plexus of vessels lying next to the ring-canal, from 

 which two vessels (a, a') pass along opposite sides of the in- 



* These vessels in Fig. 89 have been copied from Cams' Icones Zo- 

 otomicse ; ill other respects the drawing represents the anatomy of 

 P. frondosa. 



