60 THE VOYAGE OF H.M.S. CHALLENGER. 



is continuous with that of the stomodasum. In most cases the " secondary " mesenteries 

 never reach the lateral body-wall ; the " primary " mesenteries always do so. It will be 

 seen, however, that the relation of the mesenteries in such cases is precisely the reverse of 

 that found in Actiniaria. In Actiniaria a mesentery is said to be " complete " when it 

 extends from the body- wall to the stomodasum, and " incomplete " when it does not reach 

 the stomodaeum. In many Actiniaria new mesenteries are first recognisable in the angle 

 between the pedal disc and the body-wall. They are usually regarded as involutions of 

 the pedal disc and body-wall. In the genus Halcampella, for example, there are six well- 

 developed pairs of mesenteries, and in addition a variable number of rudimentary 

 " accessory " ones. " The accessory septa are small projections, which in the upper part 

 of the body alone emerge from the angle between the body-wall and oral disc." 1 Thus 

 the terms " complete " and " incomplete," as applied to the mesenteries of Zoantharia 

 generally, are not applicable in the case of Antipathidse. The behaviour of the 

 " secondary " mesenteries in that family appears to indicate that they arise as outgrowths 

 of the peristome and stomodseum, and not as involutions of the body-wall. All are 

 complete in the sense that they are united with the mesogloea of the stomodaeum. 

 Those which are incomplete become so from the fact that they lose their connection with 

 the outer wall, not with the stomodasum. 



Dimorphism. 



The dimorphism of the Schizopathinae is probably the most interesting point 

 brought out by a study of the Challenger Antipatharia. In its result it is com- 

 parable to the nutritive and sexual zooids of certain Hydroids, but its mode of pro- 

 duction is cuiite different. There appears no parallel case in the Ccelenterata so far as I 

 am aware. The gastrozooids and dactylozooids of Hydracorallinae have no resemblance, 

 the latter type being modified solely for defensive and offensive purposes. Amongst the 

 Alcyonaria dimorphism obtains in Pennatulidae, certain Alcyonidae, Pseudaxonia, &c. 

 The autozooids are of normal structure. The siphonozooids are usually without tentacles, 

 and have a well-developed siphonoglyphe ; they are usually sexless, but in certain 

 Pseudaxonia, Corallium, &c, bear ova, and apparently sometimes develop into autozooids. 

 In the Zoantharia the only case of dimorphism known to me is that described by Fowler 

 in Madrepora durvillei. In this species the dimorphism chiefly affects the structure of 

 the mesenteries. In type A the abaxial directives and the mesenteries numbered 3, 5, 

 8, and 10 (fig. 15) have a median thickening which contains a narrow canal, lined by 

 ectoderm, opening at both ends into the stomodasum. From the upper aperture the 

 canal passes somewhat horizontally into the mesenterial thickening, becomes bent 

 vertically downwards, and then turns round and takes an upward course, finally opening 



1 R. Hertwig, Challenger Actiniaria (SuppL), Zool. Chall. Exp., pt. lxxiii. (vol. xxvi.) p. 32. 



