38 THE ANTHOZOA 



Zoantharia the case is very different. The zooids present great 

 diversities of anatomical structure, even whilst their external 

 features show strong superficial resemblance to one another. We 

 have to deal with a heterogeneous instead of a homogeneous 

 assemblage of organisms ; and in spite of the labours of many 

 excellent investigators, we are still unprovided with a clue which 

 shall enable us to trace out the lines of descent of the principal 

 groups into which the sub-class must be divided. The difficulties 

 of classification are consequently great, and the arrangement here 

 adopted must be regarded as wholly provisional, though pains 

 have been taken to make it as fully as possible representative of 

 the actual state of our knowledge. 



The type form of the Zoantharia is the ordinary sea-anemone, 

 of which Actinia equina, Linn. ( = A. mesembryanthemum, Ellis and 

 Sol.), the common red anemone of our English coasts, is an excellent 

 example. 



In a common Actinia the zooid is solitary and does not produce 

 colonies by asexual generation. The animal has the form of a 

 hollow cylinder, one end of which, the base, is fixed to a rock or 

 to some other surface of attachment ; at the opposite end is the 

 mouth, surrounded by tentacles, which are arranged in several 

 circles. The following regions are easily distinguished : The 

 peristome, or space between the mouth and the bases of the 

 tentacles, the column or body wall, and the basal disc. The 

 mouth is situated in the centre of the peristome. It is elongate 

 and slit-like, and surrounded by somewhat tumid lips. In the 

 living animal the middle portion of the slit is commonly kept 

 closed by apposition of the lips, the two ends being open. The 

 tentacles are situated on the periphery and margin of the peri- 

 stome ; they are simple, digitiform outgrowths of the peristome, 

 retractile, hollow, their cavities communicating below with the 

 intermesenterial spaces of the coelenteron. Each has a small 

 aperture at its extremity. They are numerous; as many as 192 

 in adult specimens, subequal in size, arranged in four cycles of 6, 6, 

 12, 24, 48, 96. They bear a definite relation to the number of 

 mesenteries (see Fig. XIX. 1). The margin of the peristome is 

 studded with several, usually twenty -four, coloured vesicles, which 

 are batteries of nematocysts. 



The mouth opens into a tolerably long stomodaeum which, 

 like the mouth itself, is flattened from side to side. At each end 

 of the stomodaeum is a longitudinal groove, lined by specialised 

 ectoderm cells bearing long cilia. One of these grooves is termed 

 the sulcus, the other the sulculus, but they do not differ in size or 

 structure, nor is there any means of determining how the names 

 shall be applied to the two grooves in any individual specimen. 

 The mesenteries are numerous, corresponding in number to the 



