GEROULD: CAUDINA. 159 
with those of the radial vessels and with certain others which lie in 
the connective tissue on the outer side of the posterior end of the 
radial plates. 
b. Stone-canal and Madreporite. 
There is in Caudina, as in all other Molpadiidae, so far as is known, 
a single stone-canal (Plate 1, fig. 38; Plate 5, fig. 66; Plate 6, fig. 73), 
which opens into the circular water-canal in the median-dorsal inter- 
radius. It is an irregularly twisted tube embraced within the two 
layers of the dorsal mesentery; its general direction is forward and 
dorsad ; it terminates ina whitish rosette-like or heart-shaped madre- 
porite. The length of the irregular coil into which the stone-canal 
is wound is about 1.75 mm.; the total length of the canal may be 
estimated to be from four to five times as much, and its diameter is 
about 0.2 mm.— its lumen 0.06 mm. At the point where the stone- 
canal joins the circular canal (Plate 5, fig. 66) the dorsal mesentery 
is not connected with the aquapharyngeal bulb, so that the stone- 
canal and madreporite are suspended in the mesentery near its 
ventral margin; anterior to the madreporite the ventral edge of the 
mesentery encloses the genital duct. 
The madreporic body (Plate 1, fig. 8, mad.) is situated upon the 
left, or rarely upon the right, side of the tip of the stone-canal, at a 
point immediately ventral to the genital duct. It has been described 
briefly by Kingsley as rosette-shaped. The general outline of it, 
however, is not circular, but oval or often heart-shaped ; in the latter 
case the apex points forward and slightly dorsad toward the genital 
duct. It presents two surfaces, a mesenterial, which is nearly flat 
(the stone-canal arises from the posterior half of this face), and an 
antimesenterial, upon which most of the passages traversing the 
madreporite open into the body-cavity. The latter surface has only 
a slight convexity in the antero-posterior direction, whereas in the 
direction at right angles to this it is often highly convex. 
The surface is covered with meandering furrows of varying 
length,— in some cases mere pits,— at the bottom of which the outer 
openings of the pore canals are found. These channels, as I have 
determined by a plastic reconstruction, may open into the narrow 
central chamber of the madreporite without branching, or 2—4 of 
them may unite and have a common opening into the central 
chamber (compare Plate 6, fig. 74). The central chamber is flat- 
