ECHINODERMATA 237 
branch of the fork alternately, right and left, remains small 
and constitutes a pinnule, a method of branching which occurs 
in plants, and is termed 
by botanists scorpioid 
dichotomy. 
The arms and _ their 
pinnules have a grooved 
ciliated ventral surface, 
at the disk the grooves of 
the two arms: of a pair 
unite, and the five grooves 
thus formed run to the Wy 
mouth. The arms are " 
flexible, and the free Crin- 
CoO 
te =f 
oids swim through the sea 2, Ce SY &F pay 
by the graceful undula- ; aS Be 
: \s S on DS 
tions of these processes. eet iG 
. Hd, say OOS 
In atransverse section 4 ME EH 
hy 
+ 
ROR Seas 
aN 
Zon 
L 
qaqaacacs< 
co oa 
ase 
of the arm the following 
parts may be  distin- 
guished: dorsally a large 
brachial ossicle which is 
D> 
=; 
CAI 
traversed by an axial Ei g 
. se 
nerve, the contiguous We S| 
5 - : oh g 
ossicles being united and sh g 
: Sid 9 
moved bya pair of muscles HY EY & 
= DY 4 4 
(Fig. 140). Ventral to HH EE & 
. . O +4 
the ossicle is the body- Q ENS: g 
: . 7 FO 
cavity broken up _ into “ee 
four ; SPACES: which com- Fic, 139.—Pentacrinus caput Medusae. 
municate atintervals. One After Guttard, 
of these is dorsal, one ven- 
tral, and two lateral, the ventral portion is traversed by the sterile 
generative rhachis. Below these coelomic spaces lies the radial 
water-vascular vessel which gives off alternating branches to 
the nonsuctorial tube-feet. At the side of the ambulacral 
groove some spherical bodies of unknown function are situated, 
these are termed sacculi, and consist of a membrane enclosing a 
large group of spherules. 
