No. i.] MORPHOLOGY OF THE ACTINOZOA. 1 33 



has secondarily disappeared. The condition which is found in 

 the Sagartids, mentioned above, cannot be regarded as a rever- 

 sion to a more primitive condition, however. This is clearly 

 shown by the arrangement of the mesenteries. The ventral 

 surface of the body is indicated by the development of the mus- 

 culature of the first and second pairs of mesenteries (I, II), in 

 which the longitudinal muscles in bilateral types always are on 

 the ventral face, and by the development of the mesenteries V 

 and VI, which form always (in the bilateral types) in the lateral 

 and ventro-lateral intermesenterial spaces of the embryo. It is 

 evident from this that it is, as might be expected, the ventral 

 siphonoglyphe in all cases that persists, the dorsal one disap- 

 pearing. The mesenteries of the dorsal siphonoglyphe in the 

 Edwardsiae retain the original arrangement of the muscles ; it is 

 the ventral ones in which the arrangement has been modified. 

 These, in the Sagartids with a single siphonoglyphe, retain the 

 modified arrangement, while it is the dorsal ones in which a 

 secondary or tertiary modification ensues. If we imagine one 

 of these Sagartids with only the eight primary mesenteries, it 

 will be seen that the arrangement of the musculature is entirely 

 different from what is found in the Edwardsias. 



In their paper on Btmodes thallia, already referred to, Messrs. 

 G. F. and A. Y. Dixon ('89) describe an exceedingly interesting 

 observation. They found that the paired mesenteries forming 

 the second cycle of the hexamerous Actiniae are more devel- 

 oped in the lower part of the embryo than in the upper. Near 

 the base all six pairs are present at a certain stage ; a little 

 higher up only two of the pairs on each side can be found, and 

 still higher only one pair, that nearest the dorsal directives. 

 They point out that in the middle region the arrangement is 

 similar to what occurs in PeacJiia} and in the upper the arrange- 

 ment found in Gonactinia is recalled. If this difference of de- 

 velopment of these paired mesenteries indicates a primitive 

 want of simultaneity in their development, we have an impor- 

 tant clew to the explanation of the occurrence of such forms as 

 PeacJiia and Gonactinia. I have examined my preparations of 

 embryos of Aulactinia stelloidcs for indications of what the 



1 It will be seen later that the arrangement in the middle region is not the same as 

 that of Peachia, but differs from what is found there in a very important particular. 



