148 MCMURRICH. [Vol. V. 



that described by Agassiz, possessing only four tentacles. A 

 figure is given (PI. IX, Fig. 9) of the arrangement of the mes- 

 enteries in this specimen, 1 and from it will be seen that six 

 mesenteries are present. Two of these (I) are much larger 

 than the other four, and, as sections below the level of the 

 stomatodasum show, possess mesenterial filaments. It seems 

 probable, therefore, that they are the oldest mesenteries. The 

 other four mesenteries do not possess filaments, and the speci- 

 men affords no evidence as to which pair is the older. 



The next stage, however, gives the desired information. It 

 possessed six tentacles and eight mesenteries (Fig. 10). Of 

 these, those numbered I and II possessed mesenterial filaments, 

 while in III and IV they were still lacking. It is evident that 

 III and IV are the directive mesenteries, and the specimen has 

 reached the Edwardsia stage. The ventral directives (III) 

 have the same position and structure as in the preceding stage, 

 but at the dorsal surface a marked change has occurred. There 

 seems to be no room for doubt that the mesenteries marked II 

 in Fig. 1 1 are identical with those which are situated at the 

 dorsal end of the stomatodaeal axis in Fig. 12. They have, 

 however, left their original position, and migrated ventrally 

 along the stomatodaeum, while the lines of their insertion into 

 the column wall have been pushed apart by the growth resulting 

 in the formation of the third pair of tentacles (t 3 ). The fact 

 that they possess mesenterial filaments at this stage, and extend 

 a greater distance down the column wall, seems to indicate that 

 they are the second pair formed, the ventral directives being 

 the third pair, and the dorsal directives the fourth pair. The 

 sequence of development of the eight primary mesenteries of Arach- 

 nactis agrees, therefore, with what has been found in Rhodactis 

 and Manicina. 



In the youngest specimen of A. albida, described by Boveri, 

 eight tentacles were present. It would seem, therefore, that 

 the unpaired tentacle developed in the endoccel of the ventral 

 directives did not make its appearance in that species until a 

 later period. In A. brachiolata the case is different. The speci- 



1 The sections were not perfectly transverse, and the relations shown in the figure 

 were ascertained only by the study of several successive sections. Consequently the 

 figure is a composite one, and not the representation of a single section. The same 

 remark applies also to the succeeding figure. 



