10 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
rapid method. The silver appeared to have not the slightest affinity 
for the nervous elements. 
Solutions of methylen blue were at various times injected into 
the body-cavities of stupefied individuals of both Caudina and 
Synapta, and the specimens allowed to lie in sea-water for a longer 
or a shorter time. Thus far this method has likewise been of no avail. 
These two methods, so fruitful when applied to other invertebrates 
and to vertebrates, deserve a more extended trial with echinoderm 
tissue than has been given them, for the Golgi method may capri¬ 
ciously fail when employed in the study of one animal, although it 
affords excellent results when applied to a closely related form. 
2. EXTERNAL FEATURES. 
The total length of a full-grown and well-expanded specimen of 
Caudina arenata is 160-170 mm. The whole body may for con¬ 
venience be regarded as consisting of two parts, the trunk, or body 
proper, and the tail. The trunk is spindle-shaped, tapering rapidly in 
front toward the base of the tentacles and posteriorly with about the 
same rate of curvature toward the tail (Plate 4 , fig. 46). The length 
of the trunk in a specimen 170 mm. long is about 110 mm., with a 
maximum diameter of about 20 mm., whereas the tail measures about 
60 mm. in length, or in general about 85 per cent of the total length of 
the animal. The tail is differentiated from the body proper only by 
the fact that the rate of curvature of its surface from before back¬ 
wards is decidedly less than that of the posterior part of the trunk. 
Its diameter immediately behind the trunk region is 9-10 mm.; from 
this point backward it gradually diminishes in thickness and termi¬ 
nates in a truncated tip 3 mm. in diameter. Kingsley’s comparison 
of the shape of Caudina to that of an elongated pear is apt when 
applied to alcoholic specimens preserved without previous stupefac¬ 
tion, but not when applied to the living animal. 
The integument is translucent and destitute of pigment; the 
color, which depends upon the state of aeration of the blood, 
varies from pink to a purplish hue; in alcoholic specimens it 
varies from a milky white to pale brown. 
Around the mouth are grouped in a single row fifteen tentacles, 
all of equal size (Plate 1, fig. 4). Each consists of a short, nearly 
