CNIDARIA 



97 



the bilaterally symmetrical type, which at first is developed only in connec- 

 tion with budding, became so firmly established that it also found expres- 

 sion in the first stages of development from the egg (comp. above, p. 52). 

 After the formation of the first twelve tentacles, a rearrangement, 

 according to the number 6, takes place, so that there are two cycles of 

 six tentacles each. The larger ones, those of the first cycle, correspond 

 to the six primary intraseptal chambers, whereas the smaller ones, those 

 of the second cycle, alternate with them. Six large tentacles of the first 

 cycle thus alternate regularly with six smaller ones of the second cycle. 

 The appearance of new tentacles does not take place by the interpola- 

 tion of one in each of the twelve intervals between the elements of the 

 first and second cycles, but by the api^earance of six pairs, which occupy 

 only one half of these intervals, as is represented in Fig. 44 B. We 



B 



Fig. 44. — Two larva; of Actinia mesemhryanthemum ("aftei" Lacaze-Duthikbs, from 

 Balfour's Comparative Embryology) . 4, bilateral ciliated stage, with one large and 

 several small tentacle buds ; m, mouth ; B, view of an older stage fjom above. There 

 are twenty-four tentacles around the mouth. The sequence in the origin of the 

 twelve primary tentacles is a', a,b, c, d, f, e. 



here see that three tentacles lie in the intervals between every two ten- 

 tacles of the first circle, one belonging to the second cycle and two being 

 new ; but these are arranged in such a way that the middle one of the 

 three everywhere belongs to the cycle of the youngest generation. This 

 one now increases greatly in size, and outstrij^s the individuals of the 

 former second cycle, which in this way lose their rank, and are classed 

 in the third cycle. In later stages, cycles which differ in size (six ten- 

 tacles of the first, six tentacles of the second, and twelve tentacles of the 

 third cycle) actually alternate regularly with one another in position. It 

 must be observed, however, that the present third cycle does not contain 

 uniform elements, but six tentacles of the youngest stage of development 

 and six which previously belonged to the second cycle. A rearrangement 

 therefore has taken place. In the same way the number of the tentacles 

 increases from twenty-four to forty-eight and to ninety-six by the appear- 

 K. H. E. H 



