76 The Genera of the Pluinulariidce, 



which are included in the " Challenger" Report), the anterior 

 intrathecal ridge is very conspicuous, being a true partition 

 extending fully half way through the hydrotheca ; and if we 

 look into the aperture of the latter at a proper angle we see 

 that the ridge is, in most cases, even more prominent at the 

 centre than where it joins the walls of the hydrotheca, and 

 that it is often thickened at the edge and crenate. In no 

 Australian species have I seen the posterior ridge so promi- 

 nent, the only one in which I have found it well developed 

 being Aglaophenia divaricata, particularly the variety 

 which I formerly described as A. M'Coyi; and none of the 

 ten species of Halicornaria included in the " Catalogue" 

 show any trace of it. The posterior ridge, however, though 

 stated to be always wanting in the Eleutheroplea, is really 

 present in a rudimentary form in the stout variety of 

 Plumnlaria ohliqua and in P. Jilicaulis, and is well 

 developed in P. Australis, and still more in P. spinulosa. 

 The anterior ridge is large in P. producta, extending half 

 through the hydrotheca. The use of the intrathecal ridge is 

 evidently to form a protective shield, behind which the 

 hydranth can retire ; and if we consider its structure and 

 origin it will be sufficiently obvious that whether it springs 

 from the back or from the front of the hydrotheca its nature 

 is essentially the same, and that it originates from a fold or 

 constriction of the hydro thecal wall, which is more or less bent 

 upon itself either towards or away from the hydrocladium, 

 or in both directions alternately. If we take a tubular sac 

 of any flexible substance, and bend it slightly upon itself 

 near the base, we shall make a fold or crease, deepest on the 

 side towards which we bend the sac, and partially encircling 

 it. This represents the slight fold which crosses so many 

 species of Aglaophenia near the base, constituting a rudi- 

 mentar}^ posterior ridge. If we now take the same sac, 

 and bend the other extremity in the opposite direction, we 

 shall have a fold on the opposite side, and nearer the mouth. 

 This is found in many species, and varies from a slight 

 narrowing below the aperture to a deep inflection, according 

 to the degree to which the distal part is recurved. This 

 double curving is well shown in Aglaophenia longicornis 

 and in the Cladocarpus ventricosus of Allman's Gulf 

 Stream Hj^droids. It is but a step further to those forms 

 in which the recurved portion is intimately united to the 

 body of the sac, so that the double plate thus formed 

 becomes an internal partition, as seen in Aglaophenia 



