22 Edmund B. Wilson 



The fìlament is then essentially similar to the entodermic filaments of 

 Aìcyonaria. 



The truth of this surmise can ouly be tested by embryological stud- 

 ies, and it is a soiirce of the greatest regret to me that Dr. Andres's 

 very important stiidies upon the embryology of Actiniaus are stili too 

 incomplete to be decisive upon this point. I bave however bis permis- 

 sion to state that he has a certain amount of evidence that the 

 upper part ofthe filaments of the six pr luci pai septais 

 derived by a downgrowth from the ectoderm of the stomo- 

 d a e u m . If this is shown to be true by later studies it will practically 

 establish the homology bere proposed and will be a most important in- 

 dication of the descent of Aìcyonaria. 



Beider ^ came to the interesting and important conclusion that the 

 filaments of Cerianthus are entirely of ectodermic origin, on the ground 

 that they are continuous with the inner layer of the Oesophagus and 

 possess the same histological structure. But the Hertwigs justly object 

 that this conclusion cannot be accepted on histological grounds alone 

 and without knowledgc of the embryology, and it is farthermore partly 

 in direct Opposition to their Observation that the filaments of Cerianthus 

 possess the usuai three portions. The Hertwigs object farther that in 

 Sagartia parasitica the filaments bave the same structure upon the in- 

 complete septa which do not reach the Oesophagus, so that an ectoder- 

 mic origin in this case would seem to be excluded. This may be true 

 of the incomplete septa , but I cannot agree with the Hertwigs that it 

 is therefore necessarily true also of the complete septa. 



If my comparison be a just one, then the phylogenetic origin of the 

 ectodermic filaments of Alcijonaria may readily be explained. There 

 can be no doubt that the compound Alcijonaria are derived from solitary 

 forms, which probably possessed eight similar filaments. each consist- 

 ing of an ectodermic circulatory part and an entodermic digestive part. 

 As the colony-forming habit became established , bringing with it the 

 need for specialised organs of circulation, a physiological division of 

 labor took place among the filaments. In the dorsal pair the ectodermic 

 part gradually supplanted the entodermic, while the reverse process 

 took place in the other six. We might naturally expect to find some 

 record of this in the embryology, but the investigation has great inherent 

 difficulties and I bave not been able to find such a record. Possibly the 



1 Cerianthus memhranaeeus Hjiime , Ein Beitrag zur Anatomie der Actinien. 

 Sitz, der k. Akad. d. Wiss., I. Abth. März-Heft, Jahrg. 1879. 



