154 McMURRICH. [Vol. V. 



lateral and lateral pairs of secondary mesenteries which are 

 developed, whereas in Peacliia the secondary mesenteries are 

 in the ventro-lateral and lateral chambers. 



It is with the completion of this first cycle of secondary mes- 

 enteries that the tendency to develop new mesenteries on a 

 radial plan supervenes. The primary intermesenterial spaces 

 are now all occupied by paired mesenteries, and in the forma- 

 tion of new mesenteries there is an endeavor to retain this 

 symmetry ; consequently, a radial symmetry becomes superim- 

 posed upon the original bilateral plan. 



A difficulty, however, presents itself to this scheme of phylo- 

 genetic arrangement. Among the forms belonging to the genus 

 Halcampa some present a condition in which only the twelve 

 primary mesenteries are present, while others again possess in 

 addition to these six pairs of imperfect secondary mesenteries. 

 It is natural to suppose that the former, represented by such 

 species as H. clavus of Hertwig ('82), H. Kergtielensis, Hert 

 ('88), Halcampoides {Halcampa) abyssorum of Danielssen ('90) 

 are more primitive than the latter, represented by H. chrysan 

 thellum (Peach.), H. arenarea, Haddon, Halcampella maxima, 

 Hert. ('88), and probably Halcampella endromitata, Andres ('84) 

 The members of the former group differ from the larval stage 

 with only twelve mesenteries simply by mesenteries V and VI 

 being perfect, and therefore stand closer to it than do Scytopho- 

 rus, Gonactinia, or Oractis. It can hardly be supposed, however, 

 that these forms, or ones similar to them, intervene in the line 

 of descent between the two groups of Halcampa. There is too 

 great a similarity between the forms of the two groups, and too 

 great a dissimilarity between the members of either group and 

 the three forms mentioned, to allow of such a supposition. It 

 seems to me that a way out of the difficulty is to be found in a 

 reversal of the usual way of regarding the relationships of the 

 two groups. The possibility of a form presenting an apparently 

 primitive structure being the descendant of a more highly spe- 

 cialized form, must always be kept in mind, and it seems not 

 improbable that the simpler Halcampa have been derived from 

 forms possessing secondary mesenteries, instead of vice versa. 

 An analogous case is presented by the Axolotl, a form derived 

 evidently from more highly organized Urodela, but yet becom- 

 ing sexually mature, and persisting in a larval condition. The 



