BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
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tened parallel to the broad surfaces of the madreporite, so that it ma y 
be as much as 170 y wide, whereas its depth is generally much less, 
viz., 40-60 m, although in places it may be as much as 125 y deep. 
Histology. — The wall of the stone-canal (Plate 6, fig. 73) is 
composed of a thick layer of fibrous connective tissue containing 
connective-tissue cells but no muscle fibers, covered externally with 
the peritoneal epithelium of the mesentery and lined with an interna! 
ciliated epithelium; on one side of the tube this is composed of 
high, cylindrical cells, on the opposite side the cells are low and 
cubical. The transition between the two kinds of cells is gradual. 
The stone-canal of Caudina differs from that of most holothurians 
in that there are no calcareous bodies in its wall. 
The statement of Hamann, that the low, cubical cells of the internal 
epithelium are found on the side of the tube next the supporting 
mesentery, and the high, cylindrical ones on the opposite side, has 
been called in question by Ludwig (8992, p. 134), who examined 
several forms of holothurians, including Caudina, in reference to this 
matter. In studying several series of sections through the stone- 
canal of Caudina I have found that the low, cubical cells, of a height 
of only about 4 y, are uniformly on the side of the canal which is 
next the mesentery and that they increase gradually in height up to 
about 32 y on the side directly opposite, which hangs free in the 
body-cavity. Thus I can confirm Hamann's statement as far as 
Caudina is concerned, The cilia with which the tall cells are pro- 
vided are longer than those of the cubical cells. The length of the 
cilia is about equal to the height of the cells to which they belong. 
The madreporite (Plate 6, fig. 74), is composed, as in other 
holothurians, of connective tissue like that of the stone-canal. In it 
are numerous irregularly branching caleareous bodies (Plate 3, fig. 
25), which are found in greatest abundance near the surface of the 
madreporite. The arms of well-developed bodies are four or five in 
number and those of adjacent spicules interlock. These spicules 
often measure 100 y from tip to tip of the arms. Numerous spheru- 
liferous corpuscles are found in this layer of the wall, as well as in 
the channels of the madreporite, 
c. Polian Vesicle. 
The Polian vesicle (Plate 5, fig. 66, vs. Pol.) is attached to the 
circular water-canal in the left-ventral interradius. It is of an 
