ON DINOPHILUS GIGAS, 267 



homologous in the two cases. But the pharyngeal appendix 

 of Histriobdella carries three chitinous teeth^ showing that this 

 organ may in some cases develope skeletal structures; and 

 when once this is ascertained the resemblance to the MoUuscan 

 odontophore becomes obvious. Further, in Terebella, and 

 other Polychoets, the pharyngeal armature is developed from a 

 ventral and posterior diverticulum of the stomodseum 

 (fig. 14), which is apparently homologous with the correspond- 

 ing diverticulum of the Archiannelid pharynx. The wide dis- 

 tribution which some organ of this kind had among the Trb- 

 chozoa is evident from its persistence in the larvae of such 

 creatufes as Sipunculus and many others. 



It sems, therefore, legitimate to conclude that in the pha- 

 ryngeal appendix of Dinophilus and the Archiannelids we have 

 a persistent record of some ancestral organ from which deve- 

 loped the stomodaeal armature of least the Molluscs and 

 Chsetopods, and probably also of Rotifers and Crustacea. 



As for the derivation of Dinophilus and the forms which it 

 represents from simpler types, there are, as Korschelt has 

 already pointed out, many features which connect it with the 

 Rhabdocoel Turbellarians. The body cavity and excretory 

 system especially are in exactly the same condition as those of 

 a Rhabdocoel with well- developed coelomic spaces, such, for 

 example, as Mesostoma. 



It is commonly stated that myo-epithelial cells are absent 

 from the ectoderm of Rhabdocoels, and that the muscle-fibres 

 are in this group devoid of nuclei. I hope, however, shortly 

 to show that, in Convoluta at least, certain of the ectoderm 

 cells have a structure practically identical with that just 

 described in Dinophilus. 



The only characters of importance which separate Dino- 

 philus from the Rhabdocoels are, the possession of an anus, 

 and the metameric repetition of ciliated bands. Of these, the 

 second may very possibly have arisen within the limits of the 

 genus, since D. vorticoides is uniformly ciliated; but in 

 any case we have in Allostoma^ a precisely similar formation 

 ' Graff, ' Monographic der Turbellarien,' Bd. i, Taf. 19. 



