THE ANTHOZOA 43 



hood of one of the stomodaeal grooves, the sulcus, and are placed 

 right and left of it. The second pair of mesenteries (11, 11) arises 

 in the larger sulcular chamber, right and left of the sulcular 

 groove. It appears to become the sulcular directive couple of the 

 adult. The third pair of mesenteries (iir, iii) arises in the smaller 

 (sulcar) of the two original chambers, right and left of the sulcus, 

 and forms the sulcar directive couple of the adult, A fourth 

 mesenterial pair (iv, iv) is then formed, one mesentery in each 

 interspace between the first and second mesenterial pairs. There 

 is now a stage ^vith eight mesenteries which is for a short time 

 persistent. The number of mesenteries corresponds with the con- 

 dition permanent in the Alcyonaria, but the arrangement of the 

 muscle banners is quite different. The sulcular (ii, ii), sulculo- 

 lateral (iv, iv), and sulco-lateral (i, i) mesenteries have the muscle 

 banners on their s,ulcar faces ; the sulcar mesenteries (iii, in) have 

 the muscle banners on their sulcular faces. In the number and 

 arrangement of the muscles this stage exactly resembles the per- 

 manent condition in the genus Edwardsia (cf. Fig. XXI. 2). 

 The six -rayed symmetry is completed by the formation of the 

 mesenteries (v, v) in the lateral chambers, and (vi, vi) in the 

 sulco-lateral chambers, and their muscle banners are so disposed 

 that they form couples respectively with iv, iv, and i, i. 



In the genera Rhodactis, Maniciiia (a Madreporarian coral), and 

 HalrAimpa, there is an Edwardsia stage of eight mesenteries, but it 

 is arrived at somewhat differently. The mesenteries second in 

 order of formation form with the fifth the sulculo-laterul couples of 

 the adult ; the mesenteries fourth in order of formation form the 

 sulcular directives of the adult (see Fig. XX. 2). 



A third and peculiar mode of arriving at the six-rayed con- 

 dition is found in Aiptasia diaphana, which will be best understood 

 by reference to Fig. XX. 3. There is a stage with eight mesen- 

 teries, but the muscle banners on i, i, are turned in the direction 

 opposite to what occurs in Edwardsia. 



The tentacles, being placed each above an intermesenterial 

 chamber, conform in the order of their appearance and in relative 

 size to the succession of the mesenteries. When the six mesen- 

 terial couples are established, six tentacles, viz. those placed over 

 the entocoeles, become larger and longer than the six remaining 

 exocoelic tentacles ; at a later stage their sizes are equalised. 



It will readily be understood from this account, that the 

 Actinian embryo is at first bilaterally symmetrical. A divisional 

 plane passing through the sulcus and sulculus divides the body 

 into two equal and symmetrical halves, and this symmetry is pre- 

 served till the Edwardsia stage with eight mesenteries is reached. 

 With the development of the fifth and sixth pairs of mesenteries, 

 a radial arrangement is superimposed on the primitive bilateral 



