1876.] 



Structure of the Stylasterida. 



99 



the matter. Pliohotlirus is said by Pourtales* to have " occasional round 

 cavities in the centre of its branches filled with a yolk-like substance 

 contained in a membrane." These cavities seem to be ampullse; and if 

 so, then PliohotJirus may prove to belong to the Stylasteridse, and not to 

 the Milleporidse. In a specimen of Pliohothrus obtained by the ' Challen- 

 ger ' I have been able to detect neither ampullse nor tabulae. It will evi- 

 dently be possible easily to form natural genera for the Stylasteridse 

 characterized by the number of tentacles of the alimentary zooids, group- 

 ing of the tentacular zooids around them, &c. This I propose to attempt 

 when I have completed my study of the subject. 



The Milleporidse differ from the Stylasteridse in having tabulae, and in 

 possessing neither styles nor ampullae, as well as in having their mouth- 

 less zooids provided with numerous tentacles. The two families have, 

 however, many points of alliance, and they should, provisionally at least, 

 be referred to a special suborder of the Hydroidea, which may be termed 

 the Hydrocorallinae. 



A most remarkable result of the present inquiry is the determina- 

 tion that the calicles of Stylaster and Cryptolielia are tenanted and 

 formed by colonies of zooids, and not by single polyps, as was most 

 naturally hitherto supposed to be the case. Prof. Yerrill, in criti- 

 cising Prof. Agassiz's relegation of the Eugosa to the Hydroidea f, 

 dwells on the utter impossibility of Acalephs forming corals with distinct 

 septa; yet in Cryptohelia and the Stylasters septa are present in the 

 corallum, which in many cases so closely resemble those of Zoantharian 

 corals that these corals were placed by Milne-Edwards in the Oculinidae, 

 and the septa were never suspected to be pseudo-septa until Sars+ ob- 

 served that in Allopora oculina the tentacles (tentacular zooids) were 

 situate between the septa, and not upon them. I should not have de- 

 tected the compound nature of the calicular groups in Stylaster had I 

 not been led up to the fact by the examination of other genera of the 

 family, in which the tentacular zooids are widely separated from the ali- 

 mentary ones. The determination of the compound nature of the cali- 

 cular groups at once explains the otherwise very anomalous arrangement 

 of the pseudo-septa in many Stylasteridae. The condition existing has 

 been described § as a "tendency of the septate unite by their inner edges 

 and enclose in the interseptal chamber thus formed the septa of a higher 

 order." The real explanation of the matter is that the apparent inter- 

 septal chambers are the pores or calicles of the tentacular zooids. In 

 those species in which the tentacles are removed from harm's way in the 

 retracted condition of the coral by being bent inwards down into the wide 

 cavity containing the alimentary zooid (calicular cavity), these pores 

 have their walls incomplete on the side nearest to the calicle, and take 

 the form at their mouths of elongate slits, in order to allow of this inward 



* Pourtales, I. c. p. 57. 



t Prof. A. E. Verrill, ' Ann. & Mag. Nat. Hist.' 1872, 4th ser. vol. ix. p. 358. 

 t Forh. Selsk. Chr. 1872, p. 115. § Pourtales, I. c. p. 33. 



