PEDAL LACERATION IN ACTINIANS. 87 



In Fig. 9, PI. II., which represents a section through a stage 

 slightly older than that shown in Fig. 3, PI. I., the stomodeum 

 has reached nearly its normal length and, on the left side in the 

 figure, its ectodermal lining is shown extending down along the 

 border of a mesentery to form the mesenteric filament. On 

 the opposite side where the section passes through the chamber 

 between mesenteries, the ectoderm forms a flat plate extending 

 across the free border of the stomodeum. 



The Development of the Mesenteries. 



Andres (loc. cit.}, in his figures illustrating the rearrangement 

 of the mesenteries in the young fragments, shows only the pedal 

 disc and apparently understood that the mesenteries which come 

 over in the laceration piece from the parent were rearranged to 

 become the permanant mesenteries of the young actinian. He 

 makes no mention of any new growth of mesenteries or stomo- 

 deum at the oral end of the fragment as it becomes elongated 

 and assumes the characteristic shape of an actinian. 



Carlgren, 1904, 1909, takes up to a considerable length the 

 question of the arrangement of the mesenteries in laceration 

 embryos. He distinguishes four types of arrangement of the 

 first twelve mesenteries in such embryos: depending both on the 

 sequence in the appearance of the mesenteries, and on whether 

 or not any of these mesenteries, are situated in old tissue. "Was 

 erstens die Entwicklungstypus der Mesenterien in den Lacera- 

 tionstiickchen anbelangt, so haben die 1904 und die hier oben 

 mit geteilten Untersuchungen gezeugt dass es verschiedene 

 Variationen derselben giebt. Wie gross die Variation auch ist, so 

 kann mann doch deutlischerweise in dem Lacerationstiickchen , 

 d. h. in solchen Stiickchen, die einen Teil der Fussscheibe und 

 einen der Korperwand umfassen (Text Fig. I , b] , drei Verschiedene 

 Haupttypen fur die Mesenterien Entstehung unterscheiden, und 

 zwar erstens einen bilateralen mit drei primaren, vollstandigen, 

 gleich, orientierten Mesenterienpaaren (II., 4.), Zweitens ein 

 bilateralen mit nur zwei ebenfalls gleich (II., i), angeordneten 

 Paaren und schliesslich drittens einen birddialen (II., 5 and 8), 

 welche letzteren zwei Neubildungszonen enthalt, die in ihren 

 am besten entwickelten Gestalt (No. 8), Spiegelbilde zu einander 



