Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system. _Br. cart._, branchial cartilage; _M. con. str._, striated constrictor muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial; _J.v._, jugular vein; _Ep. pit._, epithelial pit.] {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 described, 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 character, 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 Ammocoetes, 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 Ammocoetes 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.