166 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in Thyone gibber (Sel.) are mentioned, and five groups of papillae in 
T. panamensis and Actinocucumis typica (Ludwig). Anal papillae are 
also described in Pelagothuria natatrix (Ludwig). 
In very few of the descriptions of the anal papillae, however, has 
it been stated whether the papillae, or groups of such processes, are 
situated in radii or interradii; but in several instances in which their 
position has been noted, they have been found at the tips of radii. 
Whether in all cases they are radial, and represent terminal tentacles 
of the radial water-vessels, can only be determined by further ob- 
servations. The structure of the papillae in Trochostoma granula- 
tum (Ludwig, 94, p. 159) reminds one at once of the conditions in 
C. arenata. In the former case the anal papillae are arranged in 
five groups, each consisting of three small flexible processes, one of 
which is longer, cylindrical in shape, and median in position ; the two 
others are shorter and situated laterally. It seems probable that we 
have here a condition similar to that in C. arenata, except that the 
papillae are longer than in the latter case, the lateral ambulacra, 
therefore, projecting beyond the surface of the body. 
2. CONTENTS OF THE WATER-VASCULAR SYSTEM. 
In the colorless, transparent, unstainable plasma of the water- 
vascular system, three sorts of cellular elements are found, viz.: 
blood corpuscles, colorless spheruliferous corpuscles, and brown 
spheruliferous cells. 
The blood corpuscles are like those found in the body cavity and 
in the blood vessels of the intestine. In color they resemble the 
blood corpuscles of vertebrates, being light yellow when seen singly 
or in small groups, crimson when massed together. They are oval, 
often being nearly circular, though sometimes much elongated, espe- 
cially in prepared sections where they are in no wise crowded; they 
may be bent in respect to their chief axis, also drawn out at one pole 
into a sharp point. From this [ conclude that they are to some extent 
capable of amoeboid movements. Specimens observed in prepara- 
tions measure on the average 8» xX 12 4,8 yp X 13.3 p being the 
dimensions of a large specimen, 7 » X 8 » those of one of minimum 
size. The nucleus is in all cases nearly spherical and about 2.7 p 
in diameter; it is highly refractive and stains deeply with haema- 
toxylin. The cytoplasm has a great affinity for eosin, but is not 
stained by haematoxylin; it has a coarsely granular appearance, 
