THE SEA- ANEMONE {Actinia) 



External Features. 



The Sea-anemone has the form of a cyhnder, the height of which sHghtly 

 exceeds the diameter. The cyhnder or column is fixed by its broad base. At 

 the free end the column passes into a horizontal plate, the disc or peristome. In 

 the middle of the disc is the slightly elevated slit-like mouth. Arising from the 

 disc, and encircling the mouth, are numerous short tentacles regularly arranged 

 in circles. There is a pore at the tip of each tentacle through which water may 

 be passed. 



.Directive mesentery Genftal organ Mesenteric filament 



^^^^^^ A. w' ^^^MjwT ^- 



~*'~*rri[ir-ii-*'— / ^^WliUULJ^"^^^ Primary mesentery 



Siphohoglypli Secondary mesentery 



Fig. 12. — A. Transverse Section through the gullet region (stomcdseum) of a Sea- 

 aneraone ; B. Transverse Section below the gullet, about the position indicated by 

 the arrow in Fig. 13, A, 



General Structure {Transverse Section). 



If a transverse section is made across the column a short way below the 

 tentacles the following structures will be seen : — 



(a) The body-wall, composed of ectoderm, mesogloea and endoderm. 



(i) The ectoderm consists chiefly of ciliated, sensory, stinging and 



gland cells, and also of numerous muscle cells, 

 (ii) The mesogloea is a tough and fairly thick supporting tissue 

 between the ectoderm and endoderm, and it forms the basis for 

 their cells. It contains amoeboid cells which have wandered 

 into it from the ectoderm chiefly ; it is therefore mesenchyme 

 tissue or incipient mesoderm, 

 (iii) The endoderm consists mainly of flagellate cells, with muscular 

 fibres at their roots. 



34 



