742 THE ANIMAL KINGDOM. 



Development is only known, and that imperfectly, in some Hexacti- 

 nians, in Cereanthus, in the Madreporian Caryophyllia and Astroides'*-. 

 It may take place externally to the parent as in Cereactis (= Actinia) 

 anrantiaca, but more generally within it, and then it is probable that the 

 ovum is fertilised in the ovary. An invaginate Gastrula is found in Actinia 

 (sp. ?) and Cereanthus, and probably in Caryophyllia. In other instances 

 (Adamsia Rondeletii= Actinia (Sagartia) parasitica ; Astroides] the endo- 

 derm probably originates by delamination from a central mass of cells as 

 in Alcyonaria. The ciliated larva is usually free-swimming and elongate, 

 sometimes with a long posterior bunch of cilia ; but in some cases (Cereactis 

 aurantiaca and occasionally in Actinia equina = A. mesembryanthemun'i) it 

 quits the parent as a young Actinian. It usually assumes the Actinian 

 condition while in the free-swimming stage. A single tentacle, correspond- 

 ing to one end of the mouth, may appear first of all as in Actinia equina ; 

 or there may be two, one at each end of the mouth, as in Heliactis (Sagartia) 

 bellis, but the number may be greater, e. g. four in Cereanthus. The twelve 

 primary mesenteries in Hexactiniae are probably developed in the following 

 order : (i) a pair at each end of the mouth, but one before the other, with 

 retractor muscles turned away from one another ; (2) a second pair, the 

 future directive mesenteries, with retractor muscles similarly disposed in 

 the intraseptal spaces of (i) ; (3) a pair on the right and left sides of the 

 mouth with .muscles also similarly disposed. The members of these two 

 lateral pairs, however, become coordinated with the members of (i), to form 

 two right and left pairs with retractor muscles turned towards one another. 

 The remaining mesenteries arise in pairs in the interseptal spaces 2 . 



The Actmiaria are found in all seas and at depths not greater than 

 2900 fathoms : and the deep sea yields a distinct fauna in which a tendency 

 to disappearance of the tentacles and to variations in structure and arrange- 

 ment of the mesenteries is observable (R. Hertwig). Some of the Madre- 

 poraria descend to great depths, e. g. Bathyactis symmetrica from 70-2900 



another bud, but if it drops off a scar remains, laying bare the coelenteric cavity of the coral. Buds 

 may develope from the sides of the scar. He has also shown that a pedunculate and fixed Flabellum, 

 his Fl. -variabile, may detach the upper part of its body. The part set free is the Fl. Stokesii or 

 Fl. Oweni (according to age) of Milne Edwards : the attached part, the Fl. spinosum, and the 

 original parent form the Fl. aculeatum of the same author. Semper mentions that in the simple 

 Fungid genus Diaseris the coral has several mouths ; that the lobes of the corallum may break away 

 to form new zooids, new lobes being developed after such a separation. He observed a Fiingia when 

 reversed, i. e. turned topsy-turvy, forming a number of zooid mouths around the edges of its base. 



Semper regards the budding of the Fungia-stock. and the pedicle in Blastotrochus as instances 

 of Alternation of Generations, the stock and the pedicle being asexual. Nothing, however, is defini- 

 tively known as to the ultimate fate of the structures in question. 



1 The Astraea of Kowalewsky appears to be the same coral as Astroides. 



2 There is considerable uncertainty as to the time, mode, &c. of appearance of the tentacles and 

 mesenteries. O. and R. Hertwig have revised (J. Z. xiii. p. 539) de Lacaze Duthiers' conclusions 

 with reference to Actinians, as stated in the text. For a summary of observations, see Balfour, Comp. 

 Embryology, i. pp. 139-143. 



