HYDRACTINIA POLYCLINA. 347 



2. Hydractinia POLYCLINA, Agassiz. 



lIvDRACTixiA POLYCLINA, — Afjassiz, Contrib. Nat. Hist. U.S., vol. iii, (il. xvi, and vol. iv, 



p. 227, pi. xxvi, fig. 18. 

 IIvDRACTiNiA EciiixATA,^ — Leichj, Mar. Invert, of New Jersey, &c., p. 3, pi. xi, fig. 35. 



TROPHOSOME. — IIydrocaulus crowded on the surface of the iiydrophyton, 

 tentacles tapering, equal or sub-equal ; margin of the colony carrying cylindrical 

 spiral appendages. Common basal expansion set with blunt conical spines traversed 

 by longitudinal jagged ridges. 



GONOSOME. — Blastostyles with conspicuous hypostome and mouth. Spoeo- 

 SACS forming a more or less dense cluster near the distal end of the blastostyle. 



Development of Gonosome. — July to February. 



Habitat. — On univalve shells inhabited by Iierniit crabs, and on rocks in tide pools. 



Batlaj metrical distribution. — From litoral to probably deep-water zone. 



Locality. — Atlantic shores of North America, Agassiz. 



It is not without great hesitation that I regard the Hydractinia polycUna of Professor 

 Agassiz as distinct from the European //. echinata. I have in vain sought in the description 

 given by Agassiz for characters sufficiently well marked to remove all doubt as to the justice of 

 the separation. The strongest arc included in the above diagnosis, which I have compiled from 

 Agassiz's description, and these will hardly be deemed altogether satisfactory. The apparently 

 more tapering form of the tentacles when extended, the greater equality in their length, and the 

 more decided development of an oral orifice in the blastostyle, are all that we can find ; and even 

 these do not agree with the characters expressed in Leidy's figure of a species which Mr. A. 

 Agassiz regards as distinct from the European form to which Leidy had referred it, and identical 

 with the Hydractinia jiolyclina of the American coast. 



Professor Agassiz informs us that the //. polyclina is not only found on the shells of 

 gasteropods inhabited by hermit crabs, but that it occurs in great abundance in tide pools, where 

 it covers the rocks " for several square feet with a rosy, velvet-like carpet, presenting a delicacy 



' It is on the authority of A. Agassiz (' Illustr. Catal.,' p. 198) that I include the Hydractinia 

 ecldnata of Leidy among the synonymes of Hydractinia polyclina. I take for granted that Mr. A. 

 Agassiz, who refers all the recorded North American species to the H. polycUna, has compared Leidy's 

 hydroid ^Tith the true H. polyclina of Prof. Agassiz, and has satisfied himself of the identity of the 

 two forms. I must confess, however, that I can find nothing either in the description or in the figur;- 

 given by Leidy to justify a belief that the H. ecldnata of this zoologist is distinct from our European 

 form, 



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