GEROULD; CAUDINA 
41 
described by Ludwig (’91 c ) and in several Molpadiidae and Cu- 
cumariidae that have been examined; or the ventral plate may be 
congruous with those of the left side and symmetrical with those of 
the right. The former I believe to be somewhat the more frequent 
condition in Caudina arenata, but the latter is of common occurrence. 
A similar variability in the ventral radial plate in the case of Anky- 
roderma musculus has been described by Ludwig (’91 a ). 
The outer and inner surfaces of each interradial plate (Plate 5 , 
fig. 64, a and b) are wedge-shaped, slightly convex and concave 
respectively. In length the interradialia measure 3-3.5 mm., in 
breadth at the anterior or broadest part 2 mm., in thickness nearly 
1 mm. The external surface is provided with two furrows in which 
lie tentacular ampullae. 
Both radial and interradial plates consist of individual spicules 
(Plate 3 , figs. 20-24), closely massed together and interlocked, the 
interstices being filled with fibrous connective tissue. There is a 
superficial layer on both axial and abaxial sides of a plate which con¬ 
sists of irregular, richly branching spicules (Fig. 20) so interlocked 
as to form a loose network. The central portion of the plate, which 
is much thicker than the superficial layer consists of long, slender, 
dichotomously, branching spicules (Figs. 21-24), which may branch 
as many as three or four times. The branches make a very small 
angle with the stem from which they arise, so that the spicule as 
a whole is much elongated. The chief axes of these spicules are 
parallel to one another and to the long axis of the radial plate. A 
branching spicule may attain a length of 300 p., whereas the stem of 
the same spicule is only 5-6 /x in diameter. 
9. WATER-VASCULAR SYSTEM. 
1. ANATOMY AND HISTOLOGY. 
The vessels of the water-vascular system in Caudina arenata con¬ 
sist of ( a ) the circular canal, (5) the stone-canal, attached to the 
circular canal in the dorsal interradius and terminating in a madre- 
poric body, (c) the single Polian vesicle, opening into the circular 
canal from behind in the left-ventral interradius, and (d) the five 
radial canals, which proceed from “ a ” anteriorly. Each of these 
five canals runs forward in the previously described groove on the 
