24 THE ANCESTRY OF VERTEBRATES 



head and trunk, emphasizing the separate origin of the 

 nervous system (cerebral and ventral ganglia) in the two. 

 HATSCHEK (1878, p. 69), as a result of his researches 

 on Polygordius, had used the word head in a somewhat 

 different sense, comprising not only the umbrella but also 

 the subumbrella of which the segmented soma is considered 

 by him to be only a kind of outgrowth or appendix. So to 

 this "head segment", corresponding to prostomium + peri- 

 stomium (first segment of the soma), not only the mouth, 

 which lies immediately beneath the prototroch, but also the 

 protonephridia and the statocysts were considered to belong 

 (cf. anon). Kleinenberg, however, emphasized that the 

 development of the larva of Polygordius, where a great 

 deal of the umbrella and the subumbrella, together with 

 the prototroch, is cast off during metamorphosis, exhibits 

 somewhat peculiar features which may easily lead to misinter- 

 pretation. In general it is quite evident in Annelidan larvae 

 that the first segment, the peristomium, containing the 

 first pair of ventral ganglia (the infra-oesophageal ganglia), 

 lies immediately behind the prototroch. The prototroch 

 accordingly must be considered as the limit of the non- 

 segmented head or prostomium and the segmented trunk, 

 and the mouth and the statocysts as belonging to the 

 latter. This view has found general acceptance among later 

 investigators of which I will mention only Salensky (1887 

 p. 632), MEYER (1890 p. 296), RACOVITZA (1896), GOODRICH 

 (1897) and ElSlG (1899, p. 226). 



Mesenchyme and coelomesoblasi. — The prostomium (a term 

 introduced by HuXLEY) contains originally the cerebral 

 ganglia which originate from its wall, though in Oligochaeta 

 they secondarily may wander backwards into the anterior 

 trunk segments. The latter contain each a pair of ventral 

 ganglia and a pair of mesoblastic somites which surround 

 a portion of the alimentary canal. The cavity of the prosto- 

 mium, however, belongs to Hatschek's primary body-cavity, 

 the blastocoele; it is, as GOODRICH (1897) remarks, "primiti- 

 vely of the nature of a blood-space, most clearly seen in 

 trochosphere larvae, where it is much enlarged." The 

 scattered mesenchymatous cells originally contained in it 

 have an ectodermal and probably a radial origin, as shown 

 by the cell-lineage investigations, whereas the trunk- or 

 coelomatic mesoderm is produced in a bilaterally sym- 

 metrical way by the two teloblasts which must be considered 



