No. i.] MORPHOLOGY OF THE ACTINOZOA. 151 



the Hexactiniae, and indications of its occurrence are to be seen 

 in the adult Scytophoms, Gonactinia, and Oractis. 



We may imagine a line of descent running from the Edward- 

 sia-like ancestors to the Hexactiniae (see diagram, Fig. IV). 

 As an offset from this line, a little below the point at which the 

 Edwardsiae stand, come the Ceriantheae. Boveri's observations 

 have shown clearly that this is the position which the Cerian- 

 theae hold with reference to the other forms. The facts which 

 I have given in the preceding pages regarding the development 

 of the mesenteries in AracJinactis indicate that they follow the 

 same sequence that is found in the Hexactiniae, and which, I 

 have endeavored to show, also occurs in Edivardsia ; and Boveri 

 has shown that structurally the eight primary mesenteries of 

 AracJinactis are equivalent to the eight mesenteries of Edward- 

 sia. These two facts allow of no reasonable explanation other 

 than that the Ceriantheae are descended from Edwardsia-like 

 ancestors. 



They do not persist in this stage, but develop a large number 

 of additional mesenteries. The formation of new mesenteries 

 is confined to a region of growth lying on the dorsal surface, 

 the first pair of secondary mesenteries being developed in the 

 space between the dorsal directives. The special region of 

 growth retains its dorsal position probably throughout the life 

 of the individual, and appears to be mainly responsible for the 

 increase in circumference of the body. A continual divarica- 

 tion of the mesenteries thus goes on, and new ones are added 

 in the growing region between the immediately preceding pairs. 

 It results from this that in the Ceriantheae the eight primary 

 mesenteries are situated in the adult on the ventral surface, the 

 secondary mesenteries lying between them and the dorsal surface, 

 with the more recent ones nearest the dorsal growing region. 



Returning to the direct line of descent, we reach a stage in it 

 in which twelve mesenteries are present. Eight of these are 

 perfect and have their longitudinal muscles arranged as in Ed- 

 ivardsia. The four additional mesenteries are imperfect and 

 arranged in two pairs, one of each pair lying on each side, be- 

 tween III and I, and I and II, and having their longitudinal 

 muscles upon the dorsal face. Corresponding to this stage no 

 adult form is known ; it is a larval stage only, but seems to rep- 

 resent a third important epoch in the development. 



