EITTER : OCTACNEMUS. 249 



Only one step further will I pursue this comparison. HypohytMus 

 is a distinctly pedunculated ascidian. This fact might be held as 

 an obstacle in the way of kinship between it and Octacnemus. Ou 

 the other hand, the relatively small and distinctly circumscribed ad- 

 hesive patch of Octacnemus might be looked upon as a remnant of the 

 peduncle. One might be warranted in speculating tliat by the principle 

 of correlated, or compensatory, growth, the great oral disc of Octacnemus, 

 with its eight arms, has been gained, in part at least, by the loss of an 

 ancestral peduncle. Possibly suggestive in this connection is the fact 

 that the peduncle of H. moseleyi Herdman, is short as compared with 

 that of H. calycodes Moseley. 



In concluding these tentative remarks on the affinities of Octacnemus, 

 it may be noted that my suggestion of relationship to HypohytMus is not 

 widely at variance from Metcalf's of its possible affinity to the Claveli- 

 nidae ; for HypohytMus and Claceliiia are certainly not remote in their 

 kinship. 



In the present imperfect state of our knowledge it would hardly be 

 profitable to enter into a detailed consideration of the relationship 

 between Metcalf's 0. ^idtagoniensis and the species now under treatment. 

 I therefore rest satisfied with pointing out the chief reasons for holding 

 that Metcalf's species should be assigned to a different genus, which, 

 however, I refrain from characterizing or naming. These are : the ab- 

 sence of the well defined and distinctly set off oral disc, and especially 

 the absence from the disc of the system of circular and radial muscle 

 bands that are so characteristic of Octacnemus ; its asexual method of 

 propagation ; its single pair of branchial stigmata ; and perhaps the 

 position of its atrial orifice. Having regard for generic distinctions as 

 they prevail generally in the Tunicata, there can, I think, be no question 

 that the characters thus indicated are sufficiently distinctive to justify 

 this proposal. 



I conclude by presenting a revision of the genus Octacnemus, and a 

 diagnosis of 0. herdmauia. 



Octacnemus Moselet, 1876. 



Body attached by a restricted, clearly defined disc, situated posteriorly 

 and A'entrally ; this disc carrying a great number of minute root filaments. 

 Anterior end differentiated into a distinct oral disc, the margin of which carries 

 eight prominent arms. 



Test gelatinous, thin, transparent. 



Mantle for the most part very delicate ; though on the dorsal side and withiu 



