348 CHartes LincoLn EDwarns, 
the calcareous ring as „slender, pliable, spongy, and in a compara- 
tively low stage of development without posterior prolongations“. 
Gonads. — In two tufts, one either side of the mid-dorsal 
mesentery. In a very young specimen (number 3, Table I), about 
5 genital tubes have sprouted from each half of the gonad anlage 
(Pl. 13, Fig. 25), but at this time the cells are primordial. 
It is only after the holothurid is much larger that sex is de- 
terminable. 
Respiratory trees. — Two respiratory trees arise from the 
anterior end of the cloaca one to the right, and one to the left, of 
the termination of the intestine. The inner wall of each tree, at 
its opening, is continuous with the enteric wall and the trees and 
the branches are fastened to the body-wall by strands of connective 
tissue. Each tree consists 6f a major, primary stem, lying in a 
lateral dorsal interradius, and of a minor, secondary stem, in & 
lateral ventral interradius. These two stems diverge from a common 
basal trunk from 3to5 mm long. The secondary is about one-third 
the length of the primary stem. The right primary stem extends 
forward to near the anterior end of the coelom while the left one 
is not quite so long. In a holothurid 9,4 cm long and 4,8 cm in 
diameter, the right primary stem gives oft 50 lateral branches, of 
which 10 are about as Jong as the secondary stem and considerably 
branched, while the rest are either simple or with few branches. 
The left primary stem has 40 lateral branches, of which 10 are 
much branched and as large as the secondary stem. This stem is 
crossed and bound down by the mesentery of the second enterie 
loop in its diagonal course anteriorly along the left dorsal interradius. 
In a very young specimen (0,7 cm long, 0,45 cm diameter), the 
primary stem (Pl. 13, Fig. 26a), the secondary stem (b), and the 
buds of 10 branches have appeared. The fine strands of connective 
tissue (c) which anchor this organ to the body-wall are to be noted. 
Levinsen, 1886, says that in a specimen 2,3 cm long, one respiratory 
tree has 38, and the other, 42 branches. | 
Muscles. — The five radial, longitudinal muscle bands are 
simple. Each band is broad and thinner along the median line so 
that sections of a transversely contracted band show a deep furrow 
toward the body-wall, and it appears almost as if the band were 
divided and the halves connected by a thin bridge of the common 
muscle substance. 
The similarly constructed introvert retractors split off from the 
