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MANUAL OF ZOOLOGY. 



(The term " actinosoma " is a very convenient one to express 

 in the Actinozoa what " hydrosoma" expresses in the Hydrozoa 

 namely, the entire organism, whether simple or compound.) 



There are three families in this section, of which the Acti- 

 nidce will require a somewhat detailed examination, since they 

 may be taken as typical of the entire class of the Actinozoa. 



FAMILY I. ACTINID.E. The members of this family are 

 commonly known as Sea-anemones, and are distinguished by 

 having no evident coralium, by being rarely compound, and 

 by having the power of locomotion. 



The body of a Sea-anemone (fig 41, a) is a truncated cone, 

 or a short cylinder, termed the " column," and is of a soft, 

 leathery consistence. The two extremities of the column are 



Fig. 41. Morphology of Actinidse. a Actinia rosea : b Arachnactis albida, 

 (After Gosse.) 



termed respectively the "base" and the "disc," the former 

 constituting the sucker, whereby the animal attaches itself at 

 will, whilst the mouth is situated in the centre of the latter. 

 Most Sea-anemones fix themselves by the base to some foreign 

 object a stone or a living animal but others (Peachta and 

 Edwardsia) bury themselves more or less completely in the 

 sand. In a few cases (Cerianthus and Peachia) the centre of 

 the base is perforated, but the object of this arrangement is 

 unknown. Between the mouth and the circumference of the 

 disc is a flat space, without appendages of any kind, termed the 

 " peristomial space." Round the circumference of the disc 

 are placed numerous tentacles, usually retractile, arranged in 

 alternating rows, and amounting to as many as 200 in number 

 in the common Actinia. The tentacles are tubular prolongations 

 of the ectoderm and endoderm, containing diverticula from 



