GEROULD: CAUDINA. 187 
of 10-15 y by perhaps 5 p, situated midway between the ends of 
the fiber, but a little to one side of its,axis. Each nucleus is sur- 
rounded by a small remnant of granular protoplasm. A single 
fiber under ordinary conditions of relaxation is from 0.6 to 0.7 
long. The cross section of a fiber has usually the shape of an 
mm, 
irregular polygon ; it is rarely circular. 
The fibers are not entirely homogeneous but ordinarily show 
a faint longitudinal striation, which is probably due to their being 
composed of fibrillae. 
A fine sareolemma encloses the fiber, and when the latter is in 
a state of contraetion this sheath is thrown into transverse folds, 
which sometimes resemble a delieate filament running spirally 
around the fiber. 
5. INNER EPITHELIUM. 
The epithelium lining the body-cavity consists of flat cells with 
polygonal outlines. They are everywhere provided with cilia. 
5. NERVOUS SYSTEM. 
The nervous system in the Molpadiidae has been investigated 
in a general way by Semper (768), Teuscher (276), Danielssen og 
Koren (78, 79), and by Jourdan (83); but later investigators, in- 
cluding Semon (83), Hamann (783, '84), and Hérouard (87, '89), 
who have paid especial attention to the histological structure of the 
nervous system in holothurians, have not extended their studies 
to the anatomical and histological conditions of this system in the 
Molpadiidae. 
In studying the nervous system in Caudina certain questions 
which apply to the whole group of Holothuroidea are naturally 
encountered, such as the connection and significance of the cellular 
elements of the central nervous system, the distribution of the 
nerves arising from the inner radial band, the number and inter- 
relation of neurons extending between the peripheral sensory cells 
and the ganglionie cells of the central nervous system, and the 
existence or absence of a circular epineural canal, 
