CCELENTERATA : ACTINOZOA. 



117 



Sub-order I. Zoantharia Malacodermata.— -In this sec- 

 tion of the Zoantharia there is either no corallum or a very ru- 

 dimentary one, in the form of a few scattered spicules. The 

 " actinosoma " is usually composed of but a single polype. 

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

 in the Adinozoa what " hydrosoma " expresses in the Hydrozoa, 

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



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

 nid(B will require a somewhat detailed examination, since they 

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



Family I. Actinid.e. — The members of this family are 

 commonly known as Sea-anemones, and are distinguished by 

 having no evident corallum, by being rarely compound, and 

 by having the power of locomotion. 



The body of a Sea-anemone (fig. 31, «) 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. 31. — Morphology of Actinidas. a Acihila rosea; b Arachitactis albida. 

 (After Gosse.) 



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

 constituting the sucker, whereby the animal attaches itself at 

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

 In a few cases ( Cerianthus and Peachid) 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 



