18 THE ANCESTRY OF VERTEBRATES 



being situated outside tiie somatic layer of the mesoderm 

 surrounding the coelom. The latter mesoderm might be 

 compared then to the lateral plate of Chordates. All this 

 is shown by fig. 4, reproduced from HATSCHEK (1878, 

 fig. 89). Also in Scoloplos I (1916. fig. 69, 70) found similar 

 figures in transverse sections and was struck by the 

 resemblance in their situation to that of the myotomes. 



The rudiment of the dorsal longitudinal musculature 

 is much less conspicuous. I feel inclined to see in the 

 rudiment of the ventral longitudinal musculature, being by 

 far the most important component of the musculature of an 

 Annelid, the beginning of the voluntary longitudinal mus- 

 culature of the trunk of Vertebrates, as suggested equally 

 by HATSCHEK (1878, p. 117). We can only suggest that the 

 dorsal longitudinal musculature of Annelids, already less 

 developed here than the ventral, has been lost in Vertebrates. 

 To the circular muscles of Annelids the musculustransversus 

 of Amphioxus shows a certain resemblance, consisting also 

 of circular muscle fibres while it is innervated by the 

 dorsal spinal nerves. The position, it is true, cannot be 

 directly compared to that of Annelids. 



Double innervation. — As to the innervation of the longitu- 

 dinal musculature of Annelids FRAIPONT (1884, p. 280-281, 

 1887, p. 36), studying Polygordius, Protodrilus and Saccocirrus, 

 observed in all these forms in the longitudinal muscles a 

 diffuse nervous plexus which is not only connected with 

 the ventral ganglion chain but also directly with cells of 

 the epidermis, as could be shown in sections and by 

 dilaceration. As a consequence of this double innervation 

 "les impressions revues de I'exterieur peuvent etre trans- 

 mises directement aux cellules ganglionnaires du plexus 

 intermusculaire, sans avoir besoin de passer par les elements 

 centraux de la moelle." In the posterior region of the body, 

 where the ventral medulla is no longer found, the second 

 mode of innervation is even the only one that is present. 

 It would be extremely interesting to know if also the 

 ectoderm of the stomodaeum possibly stands in a similar 

 relation to the intermuscular plexus. For after the stomodaeum 

 had grown out to the medullary tube of Vertebrates and 

 had extended along the neural surface of the body, similar 

 relations may easily have been established between it and 

 the contiguous lateral musculature. I feel inclined to suppose 

 that in this way the ventral spinal roots, at first in a more 



