THE PROSOMATIC SEGMENTS OF AMMOCGETES 313 
Is this prophecy borne out by the examination of Limulus? In 
the first place, these muscles were dorso-ventral and segmental, and, 
referring back to Chapter VII., Lankester arranges the segmental 
dorso-ventral muscles in three groups: (1) The dorso-ventral somatic 
muscles ; (2) the dorso-ventral appendage muscles; and (3) the veno- 
pericardial muscles. Of these the first group is represented in the 
vertebrate by the muscles which move the eye, the second group by 
the striated constrictor and adductor muscles and the muscles for the 
lower lip. There is, then, the possibility of the third group for this 
system of tubular muscles. 
Looking first at the structure of these muscles as previously de- 
scribed, so different are they in appearance from the ordinary muscles 
of Limulus, that Milne-Edwards, as already stated, called them 
“rides transparentes,” and did not recognize their muscular cha- 
racter, while Blanchard called them in the scorpion, “ligaments 
contractils,” 
Consider their attachment and their function. They are attached 
to the longitudinal sinus, according to Lankester’s observation, in such 
a way that the muscle-fibres form a hollow cone filled with blood; 
when they contract they force this blood towards the gills, and thus 
act as accessory or branchial hearts. According to Blanchard, in the 
scorpion they contract synchronously with the heart; according to 
Carlson, in Limulus they contract with the respiratory muscles. In 
Ammoccetes, where the respiration is effected after the fashion of 
Limulus, not of Scorpio, the tubular muscles are respiratory in 
function. 
Look at their limits. The veno-pericardial muscles in Limulus 
are limited by the extent of the heart, they do not extend beyond 
the anterior limit of the heart. In Fig. 70 (p. 176) two of these 
muscles are seen in front of the branchial region also attached to the 
longitudinal venous sinus, although in front of the gill-region. In 
Ammoccetes the upper limit of the tubular muscles is the group 
found in the velum; this most anterior group belongs to a region in - 
front of the branchial region—that of the trigeminal. 
Moreover, the supposition that the segmental tubular muscles 
belong throughout to the veno-pericardial group gives an adequate 
reason why they do not occur in front of the velum; for, as their 
existence is dependent upon the longitudinal collecting sinus in 
Limulus and Scorpio; which is represented by the ventral aorta in 
