l8o CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3 d Ser. 



Elytra (fig. 45) with inner margin curled up to form two funnels, the anterior 

 ear-shaped, the posterior crescentic. Elytra thin and membranaceous, very 

 slightly marked with yellow or pale green pigment; 29 to 34 pairs, borne on 

 somites 2, 4, 5, 7, 9 . . . .23, 26, 2S, 29, 31 ... .57. Elytra do not cover dorsum. 



Commensal (?) in ambulacral groove of Asterias ochracea and A. Iros- 

 chelii. 



Measurements. — Length, 41 mm.; greatest width, 7 mm.; somites (in 

 specimens of above dimensions) 60-70; the terminal ones very diminutive 

 and difficult to count.' 



With some hesitation I have identified the curious Poly- 

 noid above described as the Halosydna fragilis of Baird. 

 His description is so defective, having been made from 

 specimens which reached him in a fragmentary condition, 

 that it would be quite impossible to recognize the species, 

 were it not not for its very peculiar habitat and abortive ven- 

 tral cirri. The identity of the California specimens with 

 material recently received from Puget Sound I have made 

 out to my satisfaction ; and this circumstance heightens the 

 probability to a reasonable certainty that Baird's specimens 

 and mine belong to one and the same species. 



This species is very scarce in the neighborhood of San 

 Francisco, and I have never found it south of this point. 

 It occurs chiefly on Asterias ochracea; but even of this com- 

 mon species hardly one in fifty will be found to harbor a 

 Polyno'e frag-ilis. Northward, it is evidently far more 

 abundant. Seven young specimens from vicinity of Seattle 

 are stated by the youthful collector, John Dewhurst, to 

 whom I am indebted for them, to have come from no more 

 than a dozen starfish. 



The parasite (or commensal) does not remain constantly 

 in the ambulacral groove; sometimes it crawls upon the 

 aboral side, where its peculiar coloration harmonizes so well 

 with the colors of the starfish that it is by no means con- 

 spicuous. I have once found it on Asterias troschelii. 

 Probably it occurs on all our larger species of Asterias. 

 Whether the " parasitic worm " mentioned by Fewkes 2 



Un young specimens from Puget Sound (15-18 mm. long) the number of somites is 

 37-4o. 

 -1. c. p. 129. 



