146 McMURRICH. [Vol. V. 



have afforded the distinguished Belgian naturalist a basis for 

 his argument, that these larvae passed through a stage of long 

 duration, in which eight primary septa were present, these septa 

 corresponding to those which I have indicated in my diagrams 

 as I, II, III, and V. In other words, the macrotypal Zoanthids 

 would have an ancestry altogether different from the microtypal 

 forms, their fourth pair of primary mesenteries being equivalent 

 to the pair which Van Beneden believes to be the sixth pair to 

 develop in the microtypal forms. 



The mere fact of the occurrence of macrotypal forms seems 

 to me to indicate that the perfection of a mesentery cannot in 

 all cases be relied upon as a sure indication of its early develop- 

 ment, or, vice versa, an imperfect mesentery is not necessarily 

 one which is more recent in its appearance than all the perfect 

 mesenteries present. It seems to me exceedingly probable that 

 the imperfect condition of the dorsal directives — their position 

 with regard to the axis of the stomatodaeum and the arrange- 

 ment of their musculature shows them to be equivalent to the 

 dorsal directives of Edivardsia — is secondary, the mesenteries 

 remaining in an embryonic condition so far as their size is con- 

 cerned ; and further, there is probably a connection between 

 their imperfection and the absence of a dorsal siphonoglyphe. 



On another point also Van Beneden has been led into error 

 by attaching too great importance to slight variations in size. 

 He finds that in his larvae the mesenteries that I have indicated 

 by VI are somewhat larger than either IV or V, and therefore 

 concludes that they are older. He believes that pair VI of the 

 imperfect mesenteries has developed first, then pair IV, and 

 finally pair V. It must be noted that his conclusions are based 

 upon the study of a single specimen, and he makes no allowance 

 for individual variation. I do not find such a disparity in the 

 size of the mesenteries in the larvae which I have studied ; on 

 the contrary, all the six imperfect mesenteries are almost the 

 same size, the exceedingly slight variations which do occur hav- 

 ing no regularity. On the view that the fourth pair are retarded 

 in their development, it is easy to understand how the sixth 

 pair might surpass them slightly in size, but on account of the 

 close similarity in the arrangement of the mesenteries to what 

 we find in the larval stages of the Hexactiniae, it seems reason- 

 able to conclude that the sequence of development is the same 



