138 McMVRRICH. [Vol. V. 



retain Verrill's name, hoping to return to a consideration of the 

 true relationships of this species and those of the members of 

 Andres' family Siphonactinidae in a forthcoming report on the 

 Actiniaria dredged by the United States Fish Commission 

 steamer "Albatross" in 1887-8. 



We have then these four forms, all showing variation in the 

 manner of development of the first cycle of paired mesenteries. 

 In Scytophorus half of the dorsal pair only is developed ; in 

 Gonactinia both mesenteries of the dorsal pair are developed ; 

 in Oractis the dorsal and lateral pairs are present ; and in 

 PeacJiia the lateral and ventral pairs. A fifth arrangement 

 occurs in the Zoantheae, where the ventral pair only is devel- 

 oped at one stage of growth, the dorsal and lateral pairs never 

 appearing. The significance of these forms is, it seems to me, 

 strongly in favor of the successive development of the mesen- 

 teries of the second cycle of the Hexactiniae. It is difficult to 

 believe that all of them have been derived from the Hexactiniae 

 by degeneration. If this idea be dismissed, then we must admit 

 that in these forms portions of a complete second cycle have 

 been developed, and this fact, taken into consideration with 

 what the Messrs. Dixons have described ('89), points to the 

 successive appearance phylogenetically of the second cycle 

 mesenteries. 



III. Paractinle. 



The tribe Paractiniae was established by R. Hertwig ('82) for 

 the reception of certain forms, in which the mesenteries are 

 developed on the radial plan which is found in the Hexactiniae, 

 and have a similar association in pairs, but at the same time 

 are not arranged hexamerously. To the two forms upon which 

 Hertwig founded the tribe, viz. Sicyonis crassa, in which the 

 mesenteries are arranged octamerously, and Polyopis striata, 

 which possesses thirty-six mesenteries arranged in pairs, more 

 recent observations have added others. G. Y. and A. F. Dixon 

 ('89) have described a decamerous arrangement for Tcalia cras- 

 sicomis, and Cunningham ('89) gives an account of a similar 

 arrangement occurring in Tcalia tuberculata. Haddon has 

 shown that PeacJiia hastata is also decamerous ; while I have 

 described ('89) an octamerous arrangement in the Sagartid 

 Aiptasia annulata, and Danielssen ('90) has found the same 

 arrangement in his Sidcractis facialis. 



