GEROULD: CAUDINA. 129 
means of the alternate activity of the sphincter and the opposing 
radial muscles, The act of opening or of closing the anus during the 
respiratory process requires only about a second; the anus is gen 
erally kept open 18-20 seconds and then closed for 13-17 seconds. 
A definite period of dilation often alternates with another definite 
period of closure; there may be some irregularity in the length 
of the recurring periods, but in any event, the period of dilation 
slightly exceeds the period of closure. 
Caudina, when placed in a jar of sea-water in which no sand has 
been provided, lies upon one side of the body in a curled posture, 
the longitudinal muscles of the dorsal bivium being much contracted, 
those of the ventral trivium, relaxed; the buccal surface is turned 
downward, and the tail passes either over the head region or close 
to it in front. When cast upon the beach by the surf, Caudina 
assumes a position similar to that just described, and begins to 
burrow slowly into the sand; two or three hours may elapse before 
all of it except the tip of the tail has disappeared below the 
surface, 
4. ANATOMY AND HISTOLOGY OF THE BODY-WALL. 
The body-wall in Caudina, as in all holothurians, consists of four 
layers; these in passing from without inward are (1) an epithelium 
of columnar cells, (2) a thick layer of connective tissue, enclosing 
calcareous bodies and underlaid in the anterior part of the body by 
a layer of nerve fibers running parallel to the surface of the body, 
(3) a muscle layer, made up of five interradial areas of circular fibers 
that are interrupted at each radius and of five paired radial bands 
of longitudinal fibers, and finally (4) a thin epithelium of flattened, 
ciliated cells next to the body-cavity. 
1. EXTERNAL EPITHELIUM. 
Cuticula. The thin structureless cuticula (cta.), which covers the 
entire surface of the body and lines the pharynx and cloaca, presents 
in Caudina no peculiarities which distinguish it from that in other 
holothurians. 
The epidermis (e'th.) is composed of a single layer of columnar 
cells, which in the anterior part of the body (Plate 1, fig. 2) are about 
