98 EMBRYOLOGY 



ance of new pairs of tentacles, half of the intervals being left empty. 

 Thus by rearrangement a fourth cycle of twenty-four and a fifth one of 

 forty-eight tentacles are developed ; but these, like the third cycle before, 

 consist of elements of heterogeneous origin. 



Ordinarily the attachment of the hitherto free-swimming ciliated larva 

 takes place in the stage in which the number of tentacles is increased 

 from twelve to twenty-four. 



It is to be expected that in those forms which exhibit a special law of 

 septal growth the sequence of the appearance of the tentacles is corre- 

 spondingly modified. In a larva called Arachnactls by Sars and A. Agassiz 

 (No. 72), conditions of organization are found which, as has recently been 

 shown by C. Vogt (No. 96), connect it with the Cerianthece.^ The develop- 

 ment of the tentacles also recalls the development of the Cerianthus larva, 

 made known by Haime. In Arachnactis the tentacles do not grow out 

 in cycles between those already present, but there is a dorsal budding 

 zone (as in the case of the septa; comp. p. 95), where the youngest 

 tentacles are formed in pairs. The tentacles of the inner circle also are 

 formed in the same manner. It follows from this that the tentacles of 

 the ventral side must be the largest and oldest. The unpaired, perpetu- 

 ally dwarfed tentacle of the directive chamber, which is found between 

 the longest paired tentacles, forms an exception. 



The development of the calcareous skeleton of the Madre- 

 poraria has been studied by Lacaze-Duthiers (No. ^S) and 

 V. Koch (Nos. 83 and 85) in Astroides calycularis. It takes 

 place at the stage in which the first twelve tentacles of the 

 larva have been developed, and in which attachment usually 

 occurs. 



The calcareous skeleton is formed as a secretion on the 

 outer side of the ectoderm of the body-wall (Fig. 45). At 

 first a delicate circular basal plate arises as a secretion from 

 the ectodermal cells of the pedal disc. This hasal plate, by 

 means of which the larva attaches itself to some suitable 



^ [In regard to the development of Arachnactis, the adult form of which 

 has been found by Hertwig and Boveri, consult the recent statements of 

 E. van Beneden (No. II., Appendix to Literature on Anthozoa) and 

 BovERi (No. III., Appendix to Literature). 



A long time ago a very remarkable Actinia larva was described by 

 Semper, and recently by E. van Beneden more in detail. This larva is 

 characterized by the presence of a highly iridescent ciliate ridge running 

 lengthwise of the body. Van Beneden is inclined to refer it to the group 

 of the Zoantheae. Comp. Semper, "Ueber einige tropische Larvenfor- 

 men," Zeitschr. wisa. ZooL, Bd. xvii., 1867, and Van Beneden (No. I., 

 Appendix to Literature on Anthozoa).— H.] 



