204 ARTHUR WILLEY. 



Ctenoplana. I have proved^ as I think conclusively, that 

 the tentacle axis of Ctenoplana corresponds to the longitu- 

 dinal axis of the flat-worms. But when Ctenoplana 

 creeps, it does so, not with one of its pinnate tentacles 

 directed forwards, but with one side foremost. 



In Heteroplana as indicated in the accompanying figure, 

 the locomotion is usually conducted in a somewhat one-sided 

 fashion, and the number of marginal eyes on the 

 forwardly directed lobe is more than twice that on 

 the corresponding lobe on the opposed side of the frontal 

 region. There are no tentacles in Heteroplana. 



Apart from the presence or absence of pinnate tentacles I 

 think it is fairly evident that Heteroplana is almost directly 

 derivable from a biradial type of the same grade of organisa- 

 tion as Ctenoplana. 



But in Heteroplana the direction of locomotion (creeping) 

 has been definitely localised, and the side (namely, the right 

 side) to which the preference has been given has for that very 

 reason predominated over the other (left) side. In other 

 words, in Heteroplana there is hypertrophy of the right 

 side and atrophy of the left. 



Heteroplana is perhaps the completest novelty that I 

 have obtained. I trust that my contributions to a knowledge 

 of the primary relations of symmetry in animals in respect 

 of this form and of Ctenoplana (see ' Quart. Journ. of Micr. 

 Sci.,' vol. xxxix) will prove of value. There can be no doubt 

 that some form of symmetry is primarily present in all 

 animals from the Protozoa (amceboid animals excepted) to 

 the Vertebrata; amorphous groups like the Gastrseidse 

 (Physemariidse) and the Porifera being of a secondary or 

 derived nature. Kadial symmetry of every description seems 

 to have been preceded by what may be called polar sym- 

 metry or directive polarity, as seen in the ciliate Infu- 

 soria, the Dicyemidse, and the larvae of Actinians, &c. 



Heteroplana Newtoni was found by me on the lower 

 side of a Madrepora-stock in Sandal Bay, Lifu, on August 

 12tb, 1896. The general colour, especially of the mid-region. 



