FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 271 



single oocyte, and exhibits a distinct neck or constriction near its base, indicating 

 clearly that the sporosacs would soon have broken loose and become free-swimming. 



The sporosacs borne upon this specimen of Heterocordyle conybearei from 

 Plymouth have approximately the same form and size, and present exactly the same 

 morphological features, as our Neapolitan sporosacs ; and the trophosomes of the 

 two specimens being also in agreement, there can no longer be any doubt that 

 the specimens are specifically identical. Hence Heterocordyle conybearei, Allman, is 

 really a Dicoryne, and should therefore be known as Dicoryne conybearei (Allman). 

 To this species belong the British examples described by Allman, and the Neapolitan 

 examples * described by Weismann, and by ourselves in the earlier part of this 

 paper, f 



A Discussion of the Homology of the Sporosacs of Dicoryne. 



Several views have been advanced as to the homology of the sporosac of Dicoryne 

 conjerta. The discovery in another species of a closely related sporosac, of which 

 we have been able to trace the complete development, and the re-examination of the 

 male sporosac of D. conferta, have provided new facts on which to base a reconsidera- 

 tion of the homology of the sporosacs. 



* 



The views hitherto advanced may be grouped into two classes : — 



(1) The medusoid homology. 



(2) The polyp-homology. 



To take them in the order of their appearance : 



1. The Medusoid Homology. 



In 1861 (p. 170) Allman stated his opinion that the sporosac of D. conferta corre- 

 sponded to a normal medusiform gonophore in which certain parts were much 

 reduced. This theory he retained, and restated in somewhat greater detail in his 

 classical monograph (1872, p. 227). The significant points in his statement are as 

 follows: — "The planoblast [ = free sporosac] admits of a very instructive com- 

 parison with an ordinary medusiform gonophore. It is, in fact, a medusa in which 

 the place of the umbrella and its canals is taken by two tentacles, the manubrium of 

 the medusa being represented by the rest of the planoblast. It will be recollected 



* The species apparently extends over the northern portion of the western Mediterranean, for Mine. Motz- 

 Kossowska has recorded specimens from the Mediterranean coast of France (Banyuls) and Spain (Blanes), and from 

 the Balearic Islands (Cabrera, Mahon), in Arch. Zool. Expdr., ser. 4, tome iii, p. 76, 1905. 



t Owing to the lack of mature material indubitably belonging to 11. conybearei for comparison with our Neapolitan 

 specimens, and being faced with the definite and accepted statements of Allman and Weismann that the sporosacs of this 

 species did not become free, we considered that we were not fully justified, on the evidence available when our paper 

 was read to the Society, in regarding our Neapolitan examples as identical with H. conybearei, and as the former clearly 

 belonged to the genus Dicoryne we designated them as a new species — D. parthenopeia. (This name, which has 

 appeared only in the brief report of the meeting given in Nature, vol. xcv, p. 552, now lapses.) We suggested, how- 

 ever, that further study of the sporosacs of H. conybearei might reveal their identity with those described by us from 

 Naples. Conclusive evidence now available has shown this to be so, and accordingly the name parthenopeia has been 

 deleted from the proof. 



