REPORT ON THE ANTIPATHARIA. 57 



It appears to me, however, that neither of the foregoing explanations gives a clue to 

 the real arrangement of mesenteries in Antipathidae, and I now propose to indicate in 

 outline my own views on the subject, which appear not only to meet the case, but also 

 to throw considerable light on the homologies of the mesenteries in the Anthozoa 

 generalty. 



The arrangement of the mesenteries in pairs consisting of adjacent members, as found 

 in adult Hexactinise, does not necessarily imply that the members of a pair when at first 

 developed all bore the same relation to one another as they do ultimately. This we know 

 from the researches of Lacaze Duthiers, Kowalevsky, and the brothers Hertwig on various 

 types. Unfortunately the order of development of the first six pairs of mesenteries is 

 not thoroughly understood, and the various investigators give somewhat different 

 accounts. One point, however, appears clear, and that is that the earliest formed mesen- 

 teries are not developed on precisely the same plan as those which appear subsequently. 

 After the first twelve mesenteries have been formed, four of them situated two at each 

 end of the long axis of the stomodseurn become the pairs of " directives," and have the 

 retractor muscles on the intraseptal surfaces ; the other eight are arranged in so-called 

 " pairs," having the retractor muscles on their interseptal surfaces. It is, however, to be 

 noted that the two mesenteries forming each pair of " directives " were in point of time 

 developed together and embryologically form true pairs. The other four pairs each con- 

 sist, on the other hand, of mesenteries which are not of the same age, and therefore do 

 not come under the same category. This is clearly seen from Hertwig's figures of Peachid. 

 The first twelve mesenteries arise from single rudiments and developmentally form pairs, 

 the members of which are on opposite sides of the stomodseum. Owing to the develop- 

 ment of the retractor muscles on adjoining surfaces of the couplets along the lateral 

 walls of the stomodseum, each couplet, consisting of mesenteries of different ages, comes 

 to be regarded as a " pair." After this stage the further addition of mesenteries in 

 Hexactiniae takes place in a different manner. Buds appear on the body-wall in the 

 interseptal spaces and opposite to one another as before, but in this case each bud 

 gives rise to two mesenteries, having the retractor muscles on their adjoining surfaces. 

 In this way the majority of the mesenteries in Hexactiniae are formed, and it will be seen 

 that these " pairs," like the " directives," consist of mesenteries of the same age, but that 

 unlike them the members of a " pair " in this case were formed as adjacent and not 

 as opposite mesenteries. Perhaps two opposite pairs of this type should be considered 

 analogous to one pair of the primary mesenteries. Whatever number may ultimately be 

 present, the new pairs are always added in the lateral sections of the ccelenteron, and the 

 original " directives " are never separated, but, on the other hand, tend to become more 

 closely pressed together. As already stated, previous authors are not agreed as to the 

 order in which the first twelve mesenteries (six " pairs") are developed, and so far as the 

 Actiniaria are concerned, I do not at present propose to discuss the matter further. 



(ZOOL. CHALL. EXP. — PART LXXX. 1889.) LW 8 



