212 ZOOLOGY. 
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 Holothurians 
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 
(m) 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 at v) 
the radial vessels may be followed along the body under the 
muscles. Just above the ring-canal is situated the nervous 
ring (mr), and its radial nerves (7) can be traced along and 
outside of the radial water-vascular canals. The ampulls 
(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 
wpens between two of the tentacles on the edge of the 
mouth. 
The blood or pseudo-hemal 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. 152 have been copied from Carus’ Icones Zo- 
otomice ; in other respects the drawing represents the anatomy of 
P. frondosa. 
