180 WILLIAM F. ALLEN 



ramus ventralis. Branches from the first ramus ventraUs and 

 the second ventral motor ramus join in forming a short longi- 

 tudinal nerve, which is situated close to the cephalic ventro- 

 lateral surface of the M. cordis caudalis, and from Avhich branches 

 (R.M.C.C.) are given off to the muscle. It is likely that this 

 longitudinal nerve receives sensory fibers from the first ramus 

 ventralis, which may supply muscle spindles that are probably 

 present in the M. cordis caudalis, but which were not differ- 

 entiated with the stain used. In like manner branches from the 

 third and fourth ventral motor rami formed a more caudal 

 longitudinal nerve situated close to the lateral surface of the 

 M. cordis caudalis. The fifth ventral motor ramus is contin- 

 ued cephalad for some distance, at first along the outer surface 

 and then the inner surface of the M. cordis caudalis, &,pparently 

 furnishing the muscle with a large part of its motor supply. 

 This nerve is directly comparable with a similar nerve described 

 for the 20 cm. specimen. The sixth ventral motor ramus ends 

 in an isolated portion of the M. cordis caudalis, which at this 

 stage has not joined the main mass. 



Enough of the M. cordis caudalis {M.C.C.) has been recon- 

 structed from another adult Polistotrema (fig. 4) to show that 

 there is no conspicuous cephalic branch from one of the last 

 ventral motor rami destined to supply a large portion of the M. 

 cordis caudalis as was described for reconstructions 1 and 3. 

 The next to the last ventral motor ramus passing between the 

 myotomes and the M. cordis caudalis sends off a short caudal 

 branch (fig. 4, R.M.C.C), which follows the outer surface of 

 the M. cordis caudalis for a short distance and apparently 

 innervates it. 



In a reconstruction of the caudal heart region of a 20 mm. 

 Polistotrema embryo (fig. 12), where the M. cordis caudahs is 

 represented by a mass of myoblast derived from the embryonic 

 myotomes, two or three of the ventral motor rami, which are 

 longer than the others, have turned medially and entered the 

 M. cordis caudalis myoblast as if attracted by it. 



