ORIGIN OF METAMEEIC SEGMENTATION. 53 



(vide above p. 49) that the latter and the blastopore of 

 higher types are homologous, by the early blastopore (before 

 specialisation as the larval mouth) receives very strong support 

 from the actual structure of the Actinozooid mouth, and from 

 the newly discovered facts with regard to the history of the 

 blastopore of Peripatus capensis; and has the merit of 

 being on a priori physiological grounds easily conceivable. 



Mouth of the Actinozoa. — In the Actinozoa the mouth- 

 opening is elongated, and the animal is symmetrical on each 

 side of the long axis of the mouth. At one end of the long 

 axis the mouth is especially differentiated, and this differentia- 

 tion extends down the stomodaeum as a strongly ciliated 

 groove called by Hicksou^ the Siphonoglyphe. The cilia 

 of this groove produce a current from without inwards, while 

 the cilia of the rest of the stomodseum work in the opposite 

 direction. This differentiation of the stomodseum is particu- 

 larly conspicuous in the Hexatinian Peachia, in which there 

 is a deep strongly muscular groove along the whole length of one 

 side (the so-called ventral side) of the stomodseum (fig. 6, Si) ; 

 and the walls of the groove project at the mouth-opening beyond 

 the rest of the wall of the stomodaeum so as to form a semi- 

 circular lip conspicuous from the exterior at one end of the 

 long axis of the mouth. 



The free edges of this groove are frequently united with each 

 other, so that the groove is converted into a canal open into 

 the general cavity of the body at the lower end of the stomo- 

 daeum, and to the exterior at the mouth-opening. This junc- 

 tion of the lips of the groove seems to be simply a case of 

 adhesion, as they may with very slight effort be separated 

 without tearing the tissue. When the groove is thus con- 

 verted into a canal there are obviously two openings into the 

 body of the polyp, one through the general opening of the 

 storaodseum, and the other through this highly differentiated 

 siphonoglyphe. According to Hickson (loc. cit.) the cilia 

 work in opposite directions in these two parts of the stomo- 



1 ' Proc. Royal Soc.,' 1883. 



