92 



EMBRYOLOGY 



The twelve primary septa now arrange themselves in six 

 pairs, each of which encloses an intraseptal chamber (Fig. 42). 

 Two pairs of septa, called directive septa, lying opposite to 

 each other and corresponding in position to the angles of the 

 mouth (Fig. 42 3 and 4), bear the longitudinal muscles on the 

 sides which are turned away from each other, all other pairs 

 of septa on the sides which face each other. The gastral 

 pouch lying between any two intraseptal chambers is called 

 an interseptal chamber. New septa are never developed in 

 the intraseptal chambers. They always appear in pairs, and 

 from now on in the interseptal chambers and in cycles based 

 on the number 6. 



Fir. -10. — Diagram of the growth of the septa in Hexactinians. A, stage of 

 Manicina areolata with eight primary septa in cross-section (after H. V. Wilson) ; 1, 

 oldest pair of septa, which is in connection with the oesophagus ; ec, ectoderm; 

 en, entoderm ; s, sustentative lamella ; /, mesenterial filaments ; rf, part of the 

 ectoderm of the oesophageal tube tha'. is bent outwards and backwards at the free 

 end of the tube ; B, stage of Aulactinia stelloides with twelve primary septa (after 

 McMuerich). 



on Anthozoa) are especially important in this connection. Boveri confirms 

 the existence of both the above-mentioned types of septal growth in the 

 Hexactinia, of which the one was made known by Lacaze-Duthiers, the 

 other by Hertwig. In agreement with Haddon, McMurrich, and Dixon, 

 Boveri places special importance on the presence of an Edwardsia stage in 

 the ontogeny of the Hexactinia, and is inclined to regard the Edwardsia 

 type as the phylogenetic starting-point of all the groups of Actinia, an 

 opinion against which doubts have recently been raised, so far as re- 

 gards the Cerianthese and Zoanthese, by E. van Beneden (Nos. I. and II., 

 Appendix) and Carlgren (No. IV., Appendix). — H.] 



