ORDERACTINOIDEA. 49 



lamella? meet at a small angle and coalesce. This proceeds from 

 a slight distortion of the circular animal, and is most common in 

 species which have the lamella? undulated, by which irregularity 

 they are often brought into contact. "When the Fungia retains a 

 perfectly circular form, this union of lamellae is seldom observed. 

 It is not unusual for the lamella? in an Actinia to grow together by 

 their edges when in contact. 



b. The number of tentacles in this group, or the number of lamellae, 

 is very commonly a multiple of six. In one division of the Astra?as 

 (the Orbicella?), we find the numbers 18, 24, 36, 48 ; in other species 

 of the genus, the mode of indefinite increase and subdivision, pre- 

 vents our ascertaining how nearly they correspond. The Oculina?, 

 Dendrophyllise, and Caryophyllia? conform generally to the same 

 series, and so also the Madrepores and Antipathi, which contain 

 twelve and six tentacles respectively. 



In many instances, however, four is a submultiple, and this is 

 shown by the lobed margin of the Lucernarise, and the divisions in 

 the mouth of some Actinias. That this should often be the fact is 

 apparent from figure 15, \ 25, in which one larger and three smaller 

 intervals alternate ; and it appears that generally when a multiple of 

 six, the numbers are also multiples of four. 



While, therefore, the Alcyonaria have eight equal lamellae, the 

 Actinaria may have six or twelve equal lamella, or a number of un- 

 equal lamella?, exceeding twelve, which is a multiple of four or six. 

 The mouth and margin of some Actinia? is five lobed ; but these may 

 still conform to this principle. In the Antipathi, there are six equal 

 tentacles; in the Madreporacea, twelve tentacles, with six alternate, 

 often distinctly larger than the others : and the calcareous lamella? of 

 the cells, in other species, are usually either alternately large and 

 small, or one large alternates with three smaller, or one with five 

 smaller. 



There is, moreover, in the Actinaria, a relation between the size of a 

 polyp and the number of its internal lamella?. In many Astra?as, 

 there are fourteen or fifteen — large and small — to a breadth of a 

 quarter of an inch; and, where the number has the above relation, 

 18, 24, 36, 48, there is nearly the same relation in the diameter of the 

 cells of the coralluin. This relation admits of considerable variation, 

 which is sometimes seen to be dependent on a part of the lamella? 

 being obsolescent. The Astra?a hyades and A. pleiades both have 

 twenty-four lamella? to the cells; but in the former, which has the 



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