130 McMURRICH. [Vol. V. 



that variation in the manner of appearance of the mesenteries 

 may readily occur. It is interesting to note also that not only 

 is the relation of the newly formed mesenteries to their prede- 

 cessors different in the two cases, but likewise the arrangement 

 of the longitudinal muscles on the older mesenteries is different. 

 Boveri ('90) terms that arrangement found in Adamsia the 

 biradial type, and that of Lacaze-Duthiers the bilateral, and 

 correctly assumes that the latter, in which the arrangement of 

 the mesenterial muscles agrees with what is found in adult 

 Edwardsiae, is the original type, the biradial type having been 

 derived from it by an abbreviation of the development. The 

 symmetry of the Actinozoa, and especially of the larvae, is 

 bilateral ; in the Hexactiniae a radial arrangement of parts 

 is superposed upon this, giving what may be termed a biradial 

 arrangement ; this secondary symmetry is thrown back in the 

 ontogeny of certain forms producing the biradial type of devel- 

 opment. 



The superposition of a secondary radial symmetry upon an 

 original bilateral one is also to be seen in the Ctenophores, 

 giving rise to the characteristic biradial symmetry of these 

 forms. 



A feature of the development in the biradial types calls for 

 special mention ; that is, the reversal of the arrangement of the 

 musculature of the first pair of mesenteries. In the original 

 arrangement seen in the bilateral type, their longitudinal mus- 

 cles are upon their ventral faces and the transverse muscles on 

 the dorsal faces. In the biradial type this arrangement is re- 

 versed, so that the longitudinal muscles are upon the dorsal 

 face. 



There can be no doubt but that the arrangement of the mes- 

 enterial musculature is an important feature in determining the 

 relationships of the Actinozoa, but the occurrence of the biradial 

 type of development shows that it cannot be considered a fixed 

 and unalterable character, but must be regarded as a feature 

 which may with comparative readiness undergo modification. 

 This idea is confirmed by the occurrence of abnormalities in the 

 arrangement of the musculature in Hexactinian forms, where 

 constancy of arrangement is perhaps more to be expected than 

 in some of the less specialized groups. I have already ('89) 

 described an abnormality of this kind in Aulactinia stelloides, a 



