on the Polyzoa. 157 



the flexible stem which have the simple habit of Adeo?ieUa. 

 On the other hand, the stem is sometimes present and some- 

 times wanting in a fenestrate species, not yet described, but 

 which Mr, Busk proposed to call Adeona Gattyce. The zooe- 

 cial characters are alike in Adeona and Adeonella (restricted). 



It must be borne in mind that there is no element of struc- 

 ture amongst the Polyzoa so liable to adaptive modifications 

 as the so-called radical appendages. We meet with striking 

 illustrations of this fact amongst the Cellulariida?. In one 

 and the same species the mode of attachment and the appa- 

 ratus for effecting it exhibit the most remarkable differences. 

 In the genus Micropovella we have both crustaceous and erect 

 bilaminate forms *. In the latter section M.Jiabellarisj Busk, 

 and M. marginata^ Krauss, are attached by means of a flexible 

 stem or peduncle composed of many chitinous tubular strands ; 

 M. hastigera. Busk, a kindred species, affixes itself by an 

 adherent stony base. The calcareo-chitinous peduncle of 

 Adeona^ with its numerous radical appendages, is a much more 

 complex structure than the foregoing, in correlation with the 

 large and massive foliaceous expansions of which the zoarium 

 consists. It supplies great flexibility and great strength and 

 secure anchorage. But there is a strict analogy between the 

 two, and the greater complexity does not affect the systematic 

 significance of the structure. Genetic affinity is most surely 

 indicated by the essential characters of the individual zooecium, 

 and in these Adeona and Adeonella do not differ. That the 

 mode of attachment would vary with local circumstances and 

 the habit of colonial growth was to be expected, but the unity 

 of the group is in no degree affected by the adaptive change 

 in a mere structural detail. There is then, in my opinion, no 

 warrant for dismembering the genus Adeona ; the species com- 

 posing it will range themselves naturally and conveniently 

 under two heads : (1) with a flexible stem and (commonly) a 

 fenestrate zoarium ; (2) without a flexible stem. 



For crustaceous forms, agreeing in essential character with 

 Adeona^ Busk has instituted the genus Reptadeonella, which 

 is a return to a discredited principle of classification, and one 

 which Mr. Busk himself has abandoned in other cases. The 

 name is specially objectionable as it commemorates a discarded 



• Busk, indeed, lias instituted a genus for the latter {Fhtstramorpha) ; 

 but the only distinctive character relied upon is the erect, bilaminate 

 growth, which is absolutely immaterial. The chitinous stem and mar- 

 ginal tubes, which occur in M.fiahellaris and ili. marginata, it is admitted, 

 have no generic significance, as " a similar condition obtains in species 

 belonging to widely distinct genera" (' Challenger' Report, p. 135). It 

 is very desirable, in the interest of a natural system, that such spurious 

 genera should be weeded out. 



