3 H THE ORIGIN OF VERTEBRATES 



Ammocoetes, 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 Conns 

 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 Animoccetes 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 Eedenbaugh 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. 



