CNIDARIA 



93 



The condition described in regard to the growth of the septa applies to 

 the Hexactinia and probably to all Hexacoralla. On the other hand, there 

 are a number of groups among the Actiniaria in which other laws of 

 septal growth prevail, which furnish 

 characters of systematic importance 

 (R. Hertwig). In the Paractinice 

 (Sicyonis, Polyopsis) there are found 

 two pairs of directive septa, as in the 

 type described above, and the rest of 

 the septa also make their appearance 

 in pairs. On the other hand, the num- 

 ber of the septa is not fixed by the 

 numeral 6. The Edicardsidee (Fig. 43 

 A), like the Hexactiniag, exhibit two 

 oesophageal grooves [siphonoglyphes] 

 and two pairs of directive septa ; never- 

 theless the arrangement of the longi- 

 tudinal muscles on the septa indicates 

 a bilaterally symmetrical structure, 

 as opposed to the biradial condition 

 of the adult Hexactiniae. Of the 



eight septa present, of which only the directive pair exhibits a paired 

 grouping, six bear their longitudinal muscle bands on the side directed 



4> 



J 3 



Fig. 41.— Transverse section of a 

 young Adamsia diaphana (after O. 

 UND R. Hektwig), diagrammatic. 

 The pairs of septa 5 and 6 are iu 

 process of development. 



3 3 

 Fig. 42.— Diagram of the further growth of the septa in the Hexactiniae. Of the 

 numbers at the left 1 to 5 refer to the type of development of Adamsia (comp. Fig. 41), 

 the numbers (I.) and (FI.) to the type of development of ^ulactinia (comp. Fig. 40 B). 

 At the right, I. to IV. indicate the pairs of septa of the first to the fourth cycle ; r, 

 r, oesophageal grooves [siphonoglyphes]. 



