No. i.] MORPHOLOGY OF THE ACTINOZOA. 127 



The larvae studied by Boveri were in a corresponding stage. 



The inference to be drawn seems to be that in the Edwardsias 

 the ventro-lateral mesenteries are the first formed, and this is 

 borne out by a specimen figured by Mark (PI. XII, Fig. 2), in 

 which these are the only mesenteries present. The lines of 

 origin of two other pairs are indicated by depressions of the 

 endoderm, the depression for the dorso-lateral pair being most 

 distinct, those for the ventral pair less so, while there is no indi- 

 cation as yet of the fourth pair. It is probable that the relative 

 distinctness of the three pairs indicates the succession of their 

 development. The absence of any trace of the fourth pair of 

 mesenteries renders it uncertain as to whether they made their 

 appearance on each side of the dorsal line, or between the first 

 and third pairs. In this latter case the third pair would consti- 

 tute the dorsal directives, and the order of appearance would 

 correspond with what Lacaze-Duthiers ('72) has described, but 

 the wide separation of the mesenteries of the third pair seems 

 to indicate that the fourth pair will form the dorsal directives, 

 and the order of formation of the mesenteries will therefore 

 correspond with what H. V. Wilson (88) has described for 

 Manicina, and I ('91) for Rhodactis. 



II. Hexactinle. 



There are two points regarding the order of appearance of 

 the mesenteries of the Hexactiniae, which must be referred to 

 here. These are (1), Is it the second or fourth pair of mesen- 

 teries that is destined to become the dorsal directives ? and (2), 

 Which is the more primitive method of passing from the Ed- 

 wardsia to the Halcampa condition, — that which Boveri has 

 termed the bilateral type, or that which he designated the 

 biradial type ? 



There seems to be little doubt that the account given by 

 Kovvalewsky ('75) of the development of the mesenteries in 

 Actinia, sp. (?) is incorrect. In addition to this, two sequences 

 of formation have been described. Lacaze-Duthiers ('72) is the 

 authority for one, in which the second pair of mesenteries be- 

 come the dorsal directives, the third pair the ventral directives, 

 and the fourth pair make their appearance between the first and 

 second pairs. The correctness of this succession was called in 

 question by the Hertwigs ('79). It seemed to them more 



