168 
ME. H. N. MOSELEY ON THE ANATOMY AND 
immediately above the median digestive space, is a region devoid of eyes and 
pigment, which corresponds to a light stripe in this situation in the living 
animal. The ambulacral line is not seen here, the section being taken in 
front of its termination. The muscular fibres are seen to decussate as they 
spread out from the median line towards the periphery, both above and below 
the region occupied by the digestive tract and water-vascular space. 
A. Space in median line of body devoid of pigment and eyes. 
B. Peculiar papillae. 
E. Groups of eyes. 
P. Pigment. 
W. Water- vascular space. 
Fig. 4. Longitudinal section in the plane of the body of the anterior extremity of Bipa- 
lium Ceres , passing through the main water- vascular trunks. Drawn with the 
camera lucida. 
The preparation from which the drawing is taken was made from a speci- 
men hardened in alcohol, and the section was stained with carmine. The 
water-vascular spaces are very slightly stained, and thus stand out in relief. 
Between the two main water-vascular trunks (W, W) is seen a dark elongated 
mass, which terminates anteriorly in a pointed extremity. This mass is com- 
posed of muscles of the ambulacral line, and mainly of vertical fibres. The 
two water-vascular trunks pass up into the head, and there ramify and anas- 
tomose with one another in all directions, or rather form one large sinus 
divided up more or less into radiating channels by the vertical muscular fibres 
of the head, which appear in the drawing as dark dots. 
A. Left lateral border of anterior extremity. 
B. Right lateral border. 
W, W. Main water-vascular trunks. 
A. L. Muscles of ambulacral line. 
E, E. Eye-spots. 
D. Terminations of the diverticula of the digestive tract laid open, the 
specimen from which the section was taken having been slightly 
contorted by contraction in spirit. 
Fig. 5. Longitudinal section in the plane of the body of Bliynchodemus Thwaitesii, passing 
through the main water-vascular trunks at their anterior extremity. The pair 
of water- vascular trunks (W, W) are separated by a dark mass composed of 
the muscles of the ambulacral line ; they do not ramify at their extremities, 
but end bluntly. At the anterior extremity of the body are seen the eyes 
(E, E) in situ. 
A. L. Ambulacral line. 
W, W. Main water-vascular trunks. 
E, E. Eyes. 
