140 ACTINOZOA. 



gemmation does not occur, as in several species 

 of Actinia, the name polype is often employed to 

 denote the entire animal. 



Though the soft parts of the Actinozoa have 

 only of late years received proper attention from 

 zoologists, yet the hard structures to which these 

 animals give rise have, under the general name of 

 ** Corals," been objects of interest from a very 

 remote period. The outward aspect of Corals, as 

 preserved in our museums, is familiar to most 

 persons. Their true nature, in relation to the 

 living organisms by which they are produced, is 

 knowTi only to the student of the Actinozoa. 



The limits in size presented by the several 

 forms of Actinozoa are not very readily defined. 

 The polypes of this group are usually much larger 

 than the polypi tes of the Hydrozoa, and, in a few 

 cases, attain a diameter of even eighteen inches. 

 The gigantic dimensions of some of the coral 

 structures, produced by a combined process of 

 growth and gemmation, are well known. Though 

 the separate polypes of such a mass may, in certain 

 instances, be little larger than pin's heads, yet, 

 very often, they are half-an-inch in diameter, and 

 not unfrequently, their size is much more consider- 

 able. It can scarcely be said that any Actinozoa 

 are of microscopic dimensions. All the Ctenophora 

 are conspicuous animals ; PleurobracJiia, already 

 alluded to, one of the smaller members of the 

 group, being often about the size of an ordinary 

 marble. 



The various structures of the Actinozoa may be 

 described under the general heads of 



a. Organs of Nutrition, 

 6. Prehensile apparatus, 



