28 MORPHOLOGY. 



usually been overlooked in the description of this genus. It must, however, be admitted that 

 there are cases among the gymnoblastic hydroids in wliich the boundary can scarcely be re- 

 garded as otherwise than arbitrary. 



Besides the fresh-water Hydra, two cases have been described in which the entire hydro- 

 soma occm-s in a free state. This very exceptional condition is stated to exist in two North 

 American genera, Nemopsis and Acaulis, whose trophosomes have been found floating free in the 

 open sea. Acnulis, however, is described as becoming attached at a later ])eriod of its life.^ 



In most cases the hydrophyton becomes developed into a ramified, tree-like growth {Euden- 

 drium ramosum, Houyainvillia ramosa, Laomedea dicJiotoma, &c.). In other cases ic consists of a 

 creeping adherent, usually ramified stolon, with simple free tubes sent off fi'om it at inter- 

 vals (certain species of Clava, Campanularla, &c.) ; while sometimes, as in Hydract'mia and 

 Fodocoryne, it forms a continuous stratum, spreading over the surface of some foreign body which 

 the hydroid has selected for its al)ode. 



Nematop/iores. — In hydroids belonging to the family of Fhnimlnridm certain very remark- 

 able appendages are developed at definite and constant points from the hydrophyton. They have 

 been named nematopliores by Busk \" and though the part they perform in the economy of the 

 hydroid cannot be regarded as strictly determined, they may be here described more appropriately 

 than anywhere else. They consist each of a cup-like receptacle containing a sarcode mass, which 

 can extend itself from the cup in the form of simple or branching processes, and again completely 

 withdraw itself, so as in every respect to resemble the pseudopodial prolongations of a rhizopod. 

 There is usually a cluster of thread-cells immersed in the sarcode.^ 



The receptacle is formed of a chitinous membrane like that of the hydrothccac, and nuiy 

 either consist of a single chamber {AglaopUenia phtma) or its cavity may be divided by a 

 transverse diaphragm into a distal and a proximal chamber, which freely communicate with one 

 another through an orifice in the diapjiragm {Antemiularia). 



The nematophores may be attached to the hydrosoma by a broad base, or Idc adnate to it for 

 a greater or less extent {AylaopJienia), or they may taper away to a fine point of attachment at 

 their proximal ends {Antennularia). 



In Aylttophenia jjhima there is a nematophore always situated immediately in front of each 

 hydrotheca. It is here adnate to the walls of the hydrotheca, with whose cavity that of its own 

 receptacle communicates by means of a common lateral aperture, through which the sarcode 

 prolongations of the nematophore can freely pass into the interior of the hydrotheca. 



In the singular Campanularian genus OpJiiodes Mr. Ilincks has described certain appendages 

 which take the place of the nematophores in the nematophore-bearing genera.* They consist of 

 very extensile tentacula-like bodies, which are carried both upon the hydrocaulus and the 

 hydrorhiza. They terminate at their distal extremity in a spherical capitulum loaded with thread- 



' See M'Crady's account of Nemopsis, in Proc. Elliot, ' Soc. of Nat. Hist.' vol. i; and Stimpson 

 on Acaulis, in his 'Fauna of Grand INIanoii,' published in the Smithsouiaa Contributions. These 

 floating tropliosomes, however, arc probably only the detached hydranths of fixed forms, while the state- 

 ment that Acaulis becomes subsequently attached is almost certainly founded on an error. 



" Busk, ' Hunteriau Lectures' (MS.), delivered at the Royal College of Surgeous, London, 1857, 

 ' Figures of nematophores are given below in the section which treats of the physiology of the 



HVDROIDA. 



■' T. liiucks on Ophiodes, in ' Auu. Nat. Hist.' for Nov., 186G. 



