44 THE ANTHOZOA 



symmetry, and thenceforward the radial predominates over the 

 bilateral type, but a trace of the latter always remains in the 

 laterally compressed stomodaeum and the two couples of directive 

 mesenteries. This combination of bilateral and radial symmetry 

 has been called by Boveri (10) a biradial symmetry. 



In the genus Edwardsia, on the other hand, the symmetry is 

 permanently bilateral. 



The genus Edwardsia, of which six British species are recognised, 

 comprises small Actinians which are rounded at the aboral extremity 

 and live buried in the sand. The body is divisible into three 

 regions an upper capitulum, a median scapus, and a lower physa. 

 The capitulum and physa are retractile within the scapus, which 

 is usually invested by a friable cuticle. Though there are only 

 eight mesenteries and therefore eight intermesenterial chambers, 

 the tentacles exceed eight in number, sixteen to thirty-two are 

 generally present. A sulcus and a sulculus are both present, 

 and the arrangement of the muscle banners in the mesenteries has 

 been referred to (see Fig. XXL 1 and 2). The development 

 of Edwardsia is not known, but Boveri observed in a larva in 

 which all the eight mesenteries were present that only two of 

 them, namely, those two corresponding to the mesenteries first 

 developed in Actinia, Bunodes, etc., bore filaments. Thus it 

 seems probable that they were the first developed in Edwardsia, 

 and that the succession of mesenteries is the same in this genus 

 as in the other forms, but that in Edwardsia the development 

 stops short at the number eight, whilst the bilateral symmetry is 

 still perfect; in other forms it proceeds further, and a biradial 

 hexameral symmetry is produced. 



Seeing that most Actinians (Aiptasia is the exception) pass 

 through an Edwardsia stage, and the development of Edwardsia, 

 as far as we know it, points to the same sequence of mesenteries 

 as in Actinia, it is reasonable to conclude that the latter are derived 

 from an Edwardsia form. This conclusion is strengthened by the 

 study of the genus Halcampa, a small anemone which, like 

 Edwardsia, lives buried in the sand, and is divisible into capitulum, 

 scapus, and physa (Fig. XXI. 3). From twelve to twenty ten- 

 tacles are present (usually twelve only), and the physa is perforated 

 by about twenty-four apertures at its apex. In Halcampa chrys- 

 anthellum there are in the adult six couples of perfect mesenteries, 

 arranged on the biradiai type, and in addition six couples of very 

 small imperfect mesenteries in the exocoeles. Fig. XXI. 4 is 

 a section through the stomodasal region. Of the twelve complete 

 mesenteries six only bear gonads, viz. those which in order of 

 development are I, I ; II, II ; in, III. Below the level of the stomo- 

 daeum the asulcar directives IV, iv, are provided with filaments 

 and muscle banners, but the mesenteries v, v, and VI, vi, become 



