CCELENTERATA. 



141 





mesenteric filaments appear first of all on the primary pair of septa. 

 The increase in the number of tentacles and chambers from 12 to 24 

 has been found to take place in a very remarkable and unexpected way. 

 The law is expressed by Lacaze 

 Duthiers as follows. "The ap- 

 pearance of the new chambers is 

 not, as has been believed, a con- 

 sequence of the production of a 

 single chamber between each 

 of the twelve already existing 

 chambers, but of the birth of two 

 new chambers in each of the 

 six elements (chambers) of the 

 smaller cycle." The result of 

 this law is that a pair of tentacles 

 of the third cycle is placed in 

 every alternate space, between 

 a large and a small tentacle, of 

 the two already existing cycles, 

 which may conveniently be called 

 the first and second cycles (fig. 

 80 B). 



The twenty-four tentacles 



A 



FlG. 80. TWO STAGES IN THE DEVELOPMENT 



OF ACTINIA MESEMBKYANTHEMUM. (After Lacaze 

 Duthiers.) 



In the younger ciliated embryo A, viewed 

 from the side, only one tentacle is developed. 

 j. mouth. 



The older larva B is viewed from the face 



when 24 tentacles have just become established. 

 The letters shew the true order of succession of 

 the tentacles ; but e and / are transposed. 



formed in the above manner are 

 obviously at first very irregularly 

 arranged (fig. 80 B), but they 

 soon acquire a regular arrangement in three gi-aduated cycles of G, 6 and 12. 

 The first cycle of the six largest tentacles is the large cycle of the previous 

 stage, but the two other cycles are heterogeneous in their origin, each of 

 them being composed partly of the twelve tentacles last formed, and partly 

 of the six tentacles of the second cycle of the previous stage. 



The further law of multiplication has been thus expressed by Lacaze 

 Duthiers : " The number of chambers and still later that of the corre- 

 sponding tentacles is carried from 24 48 and from 48 96 by the birth of 

 a pair of elements in each of the 12 or 24 chambers, above which are placed 

 the smallest tentacles which together constitute the fourth or fifth cycle. 

 Since, after the formation of each fresh cycle, the arrangement of the 

 tentacles again becomes symmetrical, it is obvious that all the equal sized 

 cycles except the first are formed of tentacles entirely heterogeneous as to 

 age." 



The fixation of the free swimming larva takes place during the period 

 when the tentacles are increasing from 12 to 24. 



The general formation of the chambers in Bunodes and Sagartia is 

 nearly the same as in Actinia. 



In the two types of Actinozoa with an embolic gastrula stage the laws 

 as to the formation of the tentacles do not appear to be the same as 

 those regulating the forms observed by Lacaze Duthiers. 



In Cerianthus four tentacles are formed simultaneously at the period 

 when only four chambers are present. In Arachnitis (Edwarsia) the 

 succession of the tentacles is stated (A. Agassiz, 166) to resemble that in 

 Cerianthus. There are originally four tentacles, and at one extremity 

 of the long axis of the mouth are the oldest tentacles, while at the other 



