The mesenterial Filaments of the Alcyonarìa. 21 



indicates that they have been acquired at a recent period , and later 

 thau the entodermic filaments. Frora the nature of their Function jt Ì8 

 highly probable that they only assumed their present structure after 

 the establishment of the colony-forming habit, and we can therefore 

 hardly expect to find exactly siniilar organs in the Actinians. Through 

 the courtesy of Dr. Andres I have been enabled to examine sections of 

 Eclwarchia^ an eight-rayed solitary polyp which is in certain respects 

 intermediate in structure between the Actinians and Alcyonaria. So far 

 as conld be observed the eight filaments are quite similar, and the same 

 is true of the more typical Actinians. We should expect to find the 

 same in solitary Alcyonaria like Monoxenia^ provided these are primi- 

 tive forms. It is however quite possible that the solitary Alcyonaria 

 have lost the habit of forming colonies , and in this case we should ex- 

 pect to find the usuai arrangement as a result of inheritance. On the 

 other band, it would be most interesting to search for the analogues of 

 the ectodermic bands among the colony-forming Zoantharia. 



The Hertwig brothers have described the filaments of Actinians 

 as consisting of three portions (as Seen in transverse section^ . The 

 middle part (»Nesseldriisenstreifen«) consists largcly of »gland-cells« and 

 nettle-cells , is closely similar in general structure to the entodermic 

 filaments of Alcyonaria^ and is believed i)y the Heutwius to bc espec- 

 ially concerned in the process of digestion. The two lateral lobes 

 (»Flinmi erstreifem) , on the other band, ])ossess to the minutest detail the 

 histological structure of the ectodermic bands of Alcyonaria , and are 

 believed to be organs of circulation. 



I would suggest that itis in a high degree probable 

 that the lateral lobes or »Flimm e rstreifen«of Actinians, 

 at least in the principal or complete septa, are the homo- 

 logues of the ectodermic bands qì Alcyonaria^ and are 

 likewise ectodermic down growths fromthe stomodaeum, 

 and that the central lobes or »Nesseldrüsenstreifen« are 

 homologous with the entodermic filaments. As the Hert- 

 wiGS have described ^ , if we follow the filament upwards towards the 

 Oesophagus the central lobe disappears and only the lateral lobes 

 remain. The filament is then closely similar to the dorsal filaments of 

 Alcyonaria^ which are alwaysbilobed. Ifwe follow the filament 

 downwards, the lateral lobes disappear and the middle lobe remains. 



» Jenaischc Zeitschr. XIY, 1880, p. 5(51. 



