THE PROSOMATIC SEGMENTS OF AMMOCCETES 313 



Is this prophecy borne out by the examination of Limulus ? In 

 tlie 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 

 " brides 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 adenylate 

 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 



