94 



EMBRYOLOGY 



toward the ventral surface of the animal, whereas the ventral pair of 

 directives exhibits the longitudinal muscles on the opposite side. It 

 is worthy of consideration that, according to the coinciding observa- 

 tions of A. C. Haddon (No. 77) on Halcampa and Peachia and J. P. 

 McMuRRiCH (No. 91) on Aulactinia, the position of the muscles on the first 

 four pairs of septa agrees with the arrangement in the Edwardsidse 

 (comp. Fig. 40 B), so that accordingly in the ontogeny of some Hexactiniae 

 an actual Edwardsia stage is passed through. A bilaterally symmetrical 

 type is also developed in the gi'oups which now follow. In the Monaulea 

 (Fig. 43 B) the dorsal pair of directive septa is lacking, whereas in the 

 paired arrangement of their septa they approach the Hexactinise. The 

 Zomithece (Fig. 43 C) also exhibit a paired arrangement of the septa, but 

 each pair consists of two unequal septa : a small microseptum, not reach- 



Fig. 43.— Diagram of the position of the septa— ^, in the Edwardsidae ; B, in the 

 Monaulese : C, in the Zoantheae : D, in the Cerianthese. 



ing to the oesophagus, and a larger macroseptum, extending to the 

 oesophagus. The two pairs of directive septa constitute the only excep- 

 tion to this, the dorsal pair exhibiting only microsepta, and the ventral 

 only macrosepta. The remaining mixed pairs of septa are so arranged 

 that they fall into a dorsal and a ventral group. In the dorsal group, 

 which always consists of only four pairs, each pair turns its macroseptum 

 toward the dorsal pair of directive septa. The number of pairs of the 

 ventral group is usually considerably greater, and is increased by the 

 appearance of new pairs next to the pair of ventral directive septa (at x 

 in the two adjoining interseptal chambers). Here, therefore, only two 

 interseptal chambers function as formative seats of new pairs of septa. 



