482 K. RAMUNNf I\rKNON. 



and the condifcion already mentioned as occurring in Actino- 

 trocha. I have not observed any peculiar relation between 

 the organ under consideration and the subneural sinus. 

 There seems to be a general agreement that this structure 

 is homologous with the "notochord^' of Rhabdopleura and 

 Cephalodiscus; but whether these structures are homologous 

 with the '' Eicheldarm " of Balanoglossus remains undecided. 

 The facts mentioned above certainly support Harmer's views 

 that they are homologous with the "Eicheldarm." The 

 ciliated epithelium of the organ and its position in relation 

 to the mouth, however, indicate in Actinotrocha some 

 function connected with the ingestion of food. 



(5) The Tabular Nerve-ganglion. — The ganglion has 

 the structure desci-ibed by Masterman. The tube formed by 

 ectodermal invagination lies in the actual ganglion, and not 

 merely in front of it. The tube is lined by ciliated cells, and 

 its inner end is sacculated. This ganglion is compared to 

 the dorsal nerve-cord of the collar in Balanoglossus, which, 

 however, lies in the collar, while the ganglion of Actinotrocha 

 lies in the preoral lobe. 



Sense-organ. — This organ lies in front of the ganglion, 

 and is absent in the young larva. The ectoderm in this 

 region becomes thickened and raised up into a conical 

 papilla, and there is a development of nervous tissue at its 

 base. The cavity of the preoral lobe is continued into the 

 base of the papilla. Possibly this sense-organ represents 

 the median dorsal tentacle of the larva displaced and retarded 

 in its development. Attention may be drawn here to the 

 compound eyes which Masterman describes as occurring on 

 the tips of the tentacles in Cephalodiscus. Whatever its 

 homology may be, the organ is now a sense-organ of some 

 kind. It is curious that this organ is not developed during 

 the earlier and more active stages of the larva, but makes its 

 appearance during the later stages, and is best developed 

 when the larva becomes slow in its habits and when the need 

 for such an organ appears least. Before metamorphosis, the 

 larva^ with its ventral surface directed downwards, glides 



