An Actinian. 19 



AN ACTINIAN 

 (Metridiu m m a rgin aturn). 



External Anatomy. Observe in the expanded animal 

 that the body is made up of a cylindrical column, terminated 

 above by a more or less flattened disc and resting below 

 upon an irregularly shaped base. The column is pierced 

 with small pores, cinclides, and is limited above by a ring- 

 like fold, which contains a marginal sphincter or ring-muscle. 

 The disc is capable of considerable expansion, and may 

 present a regular number of lobes. It bears numerous rows 

 of contractile tentacles, the older and larger of which form 

 the innermost circles. These tentacles are simple outpush- 

 ings of the substance of the disc. In the centre of the disc 

 is the elongated mouth or stomodceal opening, the lateral 

 lips of which often rest against each other, and may even be 

 united, while the terminal angles are somewhat thickened 

 and form the siphonoglyphes. (In certain specimens but a 

 single siphonoglyphe is present.) The elevated area imme- 

 diately surrounding the mouth is the peristome. The base, 

 or area of attachment, is an adhesive organ, expanded later- 

 ally into a limbus, and capable of producing a considerable 

 amount of motion. In the living animal the internal struc- 

 ture is often to be seen through the transparent walls. 



Make a drawing of the specimen as seen from the side, 

 and also as seen from above. 



