314 THE ORIGIN OF VERTEBRATES 
Ammoccetes, they cannot extend beyond its limits. Now, Dohrn 
asserts that the ventral aorta terminates in the spiracular artery, 
which exists only for a short time; and, in another place, speaking 
of this same termination of the ventral aorta, he states: “Dass je 
eine vorderste Arterie aus den beiden primaren Aesten des Conus 
arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der 
Mesodermfalte des spateren Velums in die Hohe steigt um in die 
Aorta der betreffenden Seite einzumunden.” These observations 
show that the vessel which in Ammoccetes represents the longitudinal 
collecting sinus in the Merostomata does not extend further forwards 
than the velum, and in consequence the representatives of the veno- 
pericardial muscles cannot extend into the segments anterior to the 
velum. One of the extraordinary characteristics of these tubular 
muscles which distinguishes them from other muscles, but brings them 
into close relationship with the veno-pericardial group, is the manner 
in which the bundles of muscle-fibres are always found lying freely 
in a blood-space; this is clearly seen in the branchial region, but 
most strikingly in the velum, the interior of which, apart from its 
muco-cartilage, is simply a large lacunar blood-space traversed by 
these tubular muscles. 
All these reasons point to the same conclusion: the tubular 
muscles in Ammoccetes are the successors of the veno-pericardial 
system of muscles. 
If this is so, then this homology ought to throw light on the 
extraordinary innervation of these tubular muscles by the branchialis 
profundus branch of the facial nerve and the velar branch of the 
trigeminal. We ought, in fact, to find in Limulus a nerve arising 
exclusively from the ganglia belonging to the chilarial and opercular 
segments, which, instead of being confined to those segments, traverses 
the whole branchial region on each side, and gives off a branch to 
each branchial segment; this branch should supply the veno-peri- 
cardial muscle of that side. 
Patten and Redenbaugh have traced out the distribution of the 
peripheral nerves in Limulus, and have found that from each meso- 
somatic ganglion a segmental cardiac nerve arises which passes to 
the heart and there joins the cardiac median nerve, or rather the 
median heart-ganglion, for this so-called nerve is really a mass of 
ganglion-cells. In all the branchial segments the same plan exists, 
each cardiac nerve belonging to that neuromere is strictly segmental. 
