INTERRELATIONSTTTP OF THE MEIMBERS OF PEATYCTENEA. Sc) 



INTERRELATIONSHIP OF THE MEMBERS OF THE ORDER 



PLATYCTENEA; AFFINITY OF PLATYCTENEA 



TO OTHER ORDERS OF CTENOPHORA. 



The order Platyctenea has been erected by Bourne in 1900 to 

 receive the genera Coeloplana and Ctenoplana. At present it should include 

 Tjalfiella in addition and most probabl>' Gastrodes too. As to the first 

 three genera, a detailed comparative study was undertaken by Morten- 

 sen, and the reader is referred to his work in 1912. Nevertheless, the 

 following brief general review of the order would not be superfluous. 

 Of the four genera, Coeloplana and Tjalfiella are devoid of comb-plates 

 in the adult stage, while Ctenoplana and Gastrodes retain them. In the 

 tentacular apparatus, the tentacle-basis is much like that of cydippids 

 in all of the four, except that it is disposed more or less horizontally 

 instead of vertically. The secondary branches of the tentacle are present 

 in Coeloplana and Ctenoplana; in Gastrodes the tentacle is represented by 

 a short stump-like process. In the condition of the endodermal parts 

 of the gastro-vascular system, the following distinctions may be met 

 with : — In Coeloplana, eight canals representing the meridional canals 

 of ordinary ctenophores can be distinguished. In Ctenoplana, on the 

 other hand, the main part of the system appears to consist of four 

 interradially situated sac-like pouches — such a condition is found in the 

 larval stages of all ctenophores, so that Ctenoplana may be said to retain 

 the condition of that stage in the adult. In Tjalfiella, each of the 

 meridional canals is reduced into a short stalk-like diverticulum from 

 the perradial canal. Finally, in Gastrodes, the gastro-vascular canals are 

 arranged in large examples much in the same way as in ordinary 

 ctenophores, whereas, in small ones, they are represented by a connected 

 pair of cavities and each of the cavities, in a certain particular stage, 

 is laterally divided into four broad pouches, showing thus a striking 

 resemblance with the condition in a certain developmental stage of 

 Coeloplana. 



To all the four genera is common the feature that, the larval 

 pharynx is divided into two parts, of which the external part is spread 

 out and forms the ventral side of the animal. This peculiarity of the 

 pharynx constitutes, in my opinion, a feature which is the most charac- 

 teristic of the members of the Platyctenea, leading, as it does, to the 

 flatness and dorsiventrality of the body, which is to be looked upon as 

 the most important distinctive character of the group. Further, the 



