MEGAZOOIDAL INDIVIDUALS 



85 



of the body acts as a disc of attachment, muscular con- 

 traction holding the organism fast to a submarine rock or 

 shell. When the tentacles are wholly withdrawn the 

 anemone has more the shape of a little bun, a cleft at 

 the summit indicating the region of the mouth (Fig. 21, 

 a, b, c). 



Externally the Sea-anemone has the appearance of a 

 large zooid, but its internal structure reveals no zooidal 

 simplicity. The mouth does not lead into a simple body- 

 cavity as in Hydra, but into an oesophagus or gullet. This 

 passes downwards as a centrally placed tube half-way towards 

 the base of the body, its lower end opening into a wide 

 space, called the stomach, which in turn is in open com- 

 munication with a number of chambers radially situated 

 between the central oesophagus and the body-wall. The 



Fig. 21. — Actinia mesembryanthemum (tigrina), a common 

 sea-anemone, a, with tentacles half-extruded ; b, with them 

 fully extruded, and mouth visible ; c, with tentacles withdrawn. 



chambers are separated from each other by vertical septa, 

 or " mesenteries," which pass from the body-wall of the 

 anemone to the oesophageal tube, from the lower end of 

 which they are continued downwards and outwards to the 

 base of the stomach (Fig. 22). 



As in Hydra, there is an outer cell-layer of ectoderm, 

 and an inner of endoderm, the latter lining the stomach 

 and intermesenteric chambers. The gullet is lined with 

 ectoderm (Fig. 22, A, B). Between these layers there is 

 the connective tissue " mesoderm." Underneath the ecto- 

 derm there is also a layer of ganglionic nerve-cells and 

 nerve fibres, which in lesser degree are present as well at 

 the base of the endoderm. Certain of the septa are pro- 

 vided with longitudinal muscles, and it is on the free edges 



