GEROULD: CAUDINA. 169 
are blood corpuscles and spheruliferous wandering cells, the latter 
being extremely rare as compared with the former. 
The lacunae of the reproductive organs appear in the genital 
tubules as internal longitudinal projections of the wall, involving the 
connective-tissue layer and covered with a single layer of flat epi- 
thelial cells (Plate 6, fig. 83). 
Radial and tentacular lacunae. The five radial ambulacral 
vessels, as they run forward from the circular water tube, are accom- 
panied by lacunae in the connective tissue of their inner or axial walls 
(sng.7r., Plate 5, figs. 69-72). These lacunae, accompanying the 
radial vessels of the water-vascular system forward on the axial side 
of the radial plates of the calcareous ring, branch with them and 
accompany the branches as far as the tentacles and thence for a con- 
siderable distance the walls of the tentacles themselves. From the 
region of the branches to the tentacles, each radial lacuna can be 
followed backward along the entire length of a radial vessel of the 
water-vascular system; it lies in the body-wall between the loose 
fibers composing the connective-tissue partition which separates the 
hyponeural canal from the radial water canal (Plate 3, fig. 40, sag. 7r.). 
At the tip of the tail these radial lacunae communicate with a circular 
lacuna, which surrounds the anal opening. 
The contents of the radial lacunae and of the lacunae of the repro- 
ductive organs are a homogeneous plasma, staining well with eosin. 
In this plasma are occasionally found spheruliferous wandering: cells. 
Blood corpuscles, which in the intestinal vessels are far more numer- 
ous than the spheruliferous cells, are never found in the other parts 
of the lacunar system. 
11. REPRODUCTIVE ORGANS. 
1. ANATOMY. 
The sexual organs of Caudina arenata consist of two bunches of 
dichotomously branching tubules of almost uniform caliber, suspended, 
one on either side of the dorsal mesentery, at the point where they . 
unite to open into a common genital duct. The left bunch is slightly 
larger than the right. It is well known, that, when only one bunch 
of genital tubules is developed, as in many of the Holothuriidae and 
