SUMMARY 159 



the dermal musculature to the same source as that of the anne- 

 lidan vascular musculature rather than to circular or longitudinal 

 muscles. 



It is perhaps of some significance that the nerves to the 

 vascular muscles in Hirudo leave the nerve trunk in conjunction 

 with the nerves which supply only certain dorso-ventral muscles ; 

 this suggests the possibility of a common innervation of these 

 dorso-ventral muscles and the vascular musculatures. If that is 

 the case, then these dorso-ventral muscles ought to contract in 

 the presence of adrenaline. Whether any dorso-ventral muscles 

 in Hirudo contract to adrenalin I cannot yet say. My son has 

 not yet made any experiments to test this action on any dorso- 

 ventral muscles ; the muscles which gave no evidence of any 

 such contraction were the longitudinal and circular muscles of 

 Lumbricus. The dorso-ventral muscles of the leech when con- 

 tracted flatten the animal and are used when it swims through 

 the water ; they, like the longitudinal and circular muscles, are 

 paralysed by curare, and in that respect resemble the voluntary 

 muscles of the vertebrate rather than the involuntary muscles. 

 Still there is the possibility that the dorso-ventral muscles may 

 have assisted in the formation of muscles belonging both to the 

 voluntary and the involuntary nervous system in the vertebrate, 

 and helped therefore to form not only the striated segmental 

 dorso-ventral muscles but also the unstriped ectodermal muscles 

 of the somatic segmentation. If that were so I should expect to 

 find that some dorso-ventral muscles responded to adrenalin and 

 others to curare. 



On the other hand, if the dermal muscles arose independently 

 of the vascular muscles, it would be reasonable to find them repre- 

 sented in the lowest vertebrates ; but there is no sign of any in- 

 voluntary musculature under the skin in the Cyclostomes or 

 indeed so far as I know in any fishes, while the vascular muscula- 

 ture is well developed, and both sympathetic cells and chromafrine 

 cells are always to be found. 



Whatever conclusion the future may determine as to the 

 relationship between vascular and dermal muscles, it would 

 appear that certain of this group of muscles were involved in the 

 formation of the new alimentary canal, and so formed the 

 sphincters of the gut. It is possible that the dermal muscula- 

 ture assisted in the formation of the original alimentary canal 



