834 
MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
§ II. General Homologies. 
The maclreporic plate of a Holothurian is a pale, straw-coloured, hollow, conical, 
calcareous mass, lying on or near the circular canal. It may, however, be broken up 
into several portions; from each portion a canal originates, and the several canals 
generally unite to form a single stone canal. The stone canal, though sometimes 
straight, and hanging from the circular canal into the body cavity, is generally folded 
on itself, and in contact throughout the greater portion of its length with the circular 
canal. A small sinus, which sometimes exists around the stone canal, may correspond 
to the large sinus which lies in contact with the stone canal of Solaster. 
In Echinus the madreporic plate is a modified genital plate, the stone canal is a 
delicate pigmented fibrous tube, lined with cells provided with long vibratile cilia. 
This tube springs from a limited area of the plate, and runs along the “ heart ” to open 
into the circular canal at the base of the lantern. The sinus, springing from the larger 
portion of the plate, contracts into a narrow tube, and then dilates and has developed 
in its walls a lobulated glandular-looking mass, which may act the part of an excretory 
organ in connexion with the vascular system. 
In Solaster, Urcister, and Astropecten, the madreporic plate, though still placed on the 
dorsal aspect, has been removed from the genital and ocular plates by the appearance 
of antambulacral ossicles. A small calcareous stone canal and a wide membraneous 
sinus arise from the inner surface of the madreporic plate. The stone canal opens into 
a narrow circular canal; the sinus runs along under the stone canal, and, after diminish¬ 
ing considerably in size, opens into a circular blood-vessel. From this circular blood¬ 
vessel the radial (neural) vessels take their origin. A small glandular-looking mass, 
which lies in the floor of the sinus, may correspond to the glandular portion of the 
“heart” of Echinus. In Ophiura the madreporic canal springs from one of the 
interradial oral plates. Before opening into the circular canal it dilates into a vesicle. 
The wide circular canal of the Holothurian corresponds to the circular canal at the 
base of the lantern of Echinus, and to the narrow canals of Solaster, Uraster, 
Astropecten, and Ophiura. 
The long single polian vesicle of the Holothurian corresponds to the polian vesicles 
of Solaster and Astropecten, and to those of Ophiura when they are present. The 
lozenge-shaped sinuses of Holothuria have apparently nothing homologous to them hi 
either the Sea-Urchins or the Star-fish; but the large oval sinuses of Holothuria may 
correspond to the sinuses lying over the rudimentary tentacles of Echinus. In Holo¬ 
thuria the radial canals take their origin from the sinuses of the circular canal; in all 
the other types mentioned the radial canals arise directly from the circular canal. 
In Holothuria the ampullae are long and pointed; in the Star-fish they are 
spherical; in Echinus the first series are conical, while the others within the auricles 
are rounded and those without the auricles are large transversely flattened sacs. In 
Ophiura the ampullae are absent. 
