A COLLECTION OF HYDKOIDS. 335 



Systematic Position. — The present species is rather close to 

 Aglaophenia gracillima Feicl-es, from Martinique (96 

 fathoms), and described and figured by Professor Nutting.^ 

 The general character of the hydrotheca strongly resembles 

 that of parasitica; also, the corbula is described as being 

 very long, and having expanded truncated processes springing 

 from the base of the leaflets with a row of nematophores 

 situated on the top. These "truncated processes" are un- 

 doubtedly homologous with the lateral leaflets above described 

 in parasitica. 



The parasitic habit. — This species is especially interest- 

 ing on account of its close association with the coralline sea- 

 weed to which it is attached. The hydrorhiza creeps on the 

 surface of the alga (text-fig. 17, C), and at some little distance 

 behind the growing point of the stolon, sucker-like outgrowths 

 are produced, which grow down into the tissue of the alga. 

 These suckers are shown in section in fig. D, and also in 

 PL XLYIII, fig. 32. The sucker is composed of a modified 

 clump of ectoderm cells, which have become exceedingly 

 elongated. The perisarc of the hydrorhiza is divided into an 

 outer (^]) and inner layer {'p^), and neither of these layers 

 are generally continued down for any distance into the pit 

 formed by the sucker, although occasionally such may be the 

 case. Usually there are two rows of suckers on the under 

 surface of the hydrorhiza (fig. C). 



In fig. 32 of the plate it can be seen that the elongated, 

 modified, ectoderm cells of the sucker are very granular, and 

 they are shown perforating (p.c.) and entering the cells of the 

 alga. Generally the sucker stretches down through the cortex 

 (0) to the medulla (ill) of the alga, and here the sucker-cells 

 appear to retain a direct contact with the lining tissue of the 

 alga without a layer of perisarc being formed. It is extremely 

 probable that the hydroid extracts substances of use to it from 

 the alga, and that the numerous suckers are not merely for 



' Nutting, C. C, -American Hydroids." Part I, "The Phiniulariida?,"' 

 1900, p. ]03. 



