172 ACTINOZOA. 



differs, in no essential respect, from that of the 

 outer boundary of the body. 



The polype, while yet immature, presents a well- 

 marked bilateral symmetry. The oral fissure is 

 produced more in one direction than in another, 

 its form being by no means, as some have wrongly 

 stated, circular. At its opposite angles, gonidial 

 grooves, in certain cases one only, arise. Two of 

 the mesenteries in Actinia, as Haime has pointed 

 out, are developed opposite to each other before 

 the rest make their appearance, and these in direc- 

 tion correspond with the two mouth-angles. The 

 mesenteries grow from above downwards and, in 

 some long-bodied polypes, do not extend much 

 farther than the level of the free end of the 

 digestive sac, or, becoming narrowed and much 

 convoluted, are finally lost in the proximal portion 

 of the wall of the somatic cavity. In Cerianthus 

 two of the mesenteries descend, far below the 

 others, almost to the orifice at the base of the 

 general cavity. The remaining mesenteries, much 

 shorter than the preceding, gradually diminish in 

 length till they reach two points at either side of 

 the larger mesenteries and, like them, opposite to 

 one another. 



The rudimentary tentacles, also, afford proofs of 

 the symmetry just noticed. In those young Zoan- 

 tharia which possess five of these appendages, 

 four, as Agassiz has stated, are arranged in pairs 

 on either side of the mouth, while the fifth lies 

 opposite one of the oral angles. 



The subsequent development of the tentacles 

 has been well illustrated by Haime from the case 

 of the common Sea-anemone. The succession of 

 these organs is effected from within outwards in a 



