4IO 



COELENTERATA ANTHOZOA 



The stomodaeum exhibits a single long deep siphonoglyph, 

 which is probably dorsal in position. 



There are two tentacles to each inter-mesenteric space, one 

 being marginal and the other cireumoral. The gonads are borne 

 upon alternate mesenteries, and both ova and spermatozoa are 

 produced by the same individual. 



The ectoderm of Cerianthus is remarkable for the immense 

 number of nematocysts and gland cells. The latter secrete a 



M^^m 



Fig. 179. — Cerianthus membranaceus. Colour pink, with tentacles annulated pink and 

 brown. About 35 cm. in length. (After Andres. ) 



quantity of mucus which binds the threads of the discharged 

 nematocysts into a sticky feltwork and this secures particles of 

 sand and mud, the whole forming a long tube in which the animal 

 freely moves. This tube is often of considerable thickness. It 

 is tough and resistant, smooth inside but ragged and muddy 

 outside. It is often many times the length of the animal's body. 

 The embryo of Cerianthus is set free before the completion of 

 segmentation, and it gives rise to a floating pelagic larva known 

 as Arachnactis. It has a variable number of tentacles and 

 mesenteries according to its age, but when it reaches a size of 



