RELATION OP THE NEMERTEA TO THE VERTEBRATA. 625 



between three longitudinal sterns^ any remnants of the lateral 

 cords are yet detectable in the Vertebrate embryos, perhaps 

 even in the adults, I am inclined to answer in the affirmative. 

 Here I must be allowed to insert a reference to the three 

 figures on PI. XLII, which will facilitate the exposition of the 

 further consequences of the hypothesis I am here developing 

 Fig. 1 represents the chief points in the nervous system of the 

 Nemertea. The brain-lobes are simple lateral swellings of the 

 longitudinal stems, as in Carinella; plexus, medulla, and 

 transverse stems, together with brain-lobes and lateral stems, 

 may be considered as forming part of the integument (cf. 

 Carinina). A double innervation of the respiratory portion 

 of the intestine is indicated ; one due to visceral branches 

 {vi. sy) springing from the plexus (or from its transverse 

 tracts), the other to the more specialised nerve (w), which has 

 above been indicated as the Nemertean vagus nerve. The 

 plexus and its innumerable radial fibres, both sensory and 

 motor, are not indicated in this figure, nor are the nerve-stems 

 which, when present, pass from the lateral stems directly to 

 the integument. 



This figure must now be compared with the two others. Of 

 these, PI. XLII, fig. 2, diagrammatically represents the chief 

 points that may be considered as characteristic of the nervous 

 system of a lower Vertebrate, in which the dorsal and ventral 

 roots of the spinal nerves {dr and vr) are as yet separate nerve- 

 tracts, in which the sympathetic nerve system is as yet only 

 represented by visceral branches given off by these dorsal roots 

 {yi. sy), and in which the polymerous character of a primitive 

 vagus (Vag) is established. 



PI. XLII, fig. 3, stands for Amphioxus, as far as we know 

 its nervous system, more particularly through the researches of 

 Rohon and others. It diff'ers from the foregoing by the 

 absence of a distinct brain swelling and of a polymerous vagus. 

 A number of spinal nerves are considered as homologous with 

 the vagus of Vertebrates by Rohon. The commissural con- 

 nections between the successive spinal nerves form a plexus, 

 which is peripherally even much more elaborate, according to 



