No. i.] MORPHOLOGY OF THE ACTINOZOA. 147 



as in that group, especially as the dissimilarities are explicable. 

 The fifth and sixth pairs of the Hexactiniae appear to arise 

 simultaneously, and it is possible that in individual cases one 

 pair may originate a little before the other, and that the sixth 

 pair in Van Beneden's larvae may have been slightly older than 

 the fifth pair ; but even so, such a sequence of development 

 could not be regarded as having any phylogenetic significance, 

 but would merely indicate an individual variation. 



It is upon the two erroneous ideas here discussed that Van 

 Beneden bases his views as to the affinities and phylogeny of 

 the Zoantheas. He believes that they have never had any con- 

 nection with the forms represented by the genus Edwardsia or 

 with Edwardsia-\\ke. ancestors, but that they are a perfectly inde- 

 pendent branch. All other groups of Actinians, however, stand 

 in phylogenetic relation to the Edwardsiae, and hence there is 

 no relation between the Zoantheas and the other Actiniaria, not- 

 withstanding the many points which the two groups have in 

 common. Since this idea, however, is based on an erroneous 

 interpretation of facts, it does not require further discussion. 



V. Cerianthe,e. 



It has been shown by the Hertwigs ('79). Vogt (88), and 

 H. V. Wilson (88) that in the later stages of Cerianthus new 

 mesenteries are formed only in the chamber between the dorsal 

 pair of mesenteries ; and Boveri ('90) has demonstrated the 

 same fact for Arachnactis. Concerning the sequence of devel- 

 opment of the first eight mesenteries, however, we have no 

 direct observations, though from Boveri's discovery of the simi- 

 larity in the arrangement of the musculature of the eight pri- 

 mary mesenteries to what is found in the Edwardsias and in the 

 Hexactiniae, it seems probable that their order of development 

 is the same as in the latter group. 



I have been able to throw some definite light upon this point 

 from the study of some young specimens of Arachnactis brachio- 

 lata obtained at Wood's Holl. A. Agassiz (62) has given an 

 account of the external characters of larvae of this form and of 

 the development of the tentacles, but has given no account 

 of the formation of the mesenteries. 



The youngest specimen I obtained was in the same stage as 



