82 MARGARET 0. COLLCUTT. 



An injured part of the hydrophytou repairs the injury by 

 a network of coenosarcal tubes invested by a chitinous perisare, 

 the meshes of which are ultimately obliterated by the thicken- 

 ing and coalescence of their chitinous walls. The ectoderm 

 and endoderm of the hydranths is continuous with the ecto- 

 derm and endoderm of the basal coenosarc. 



The spiral zooids are long, cylindrical, mouthless hydroids, 

 with a crown of rudimentary tentacles crowded with large 

 nematocysts ; they have a tubular eudodermal cavity, and 

 possess the power of coiling themselves into a spiral : the 

 mesoglcBa fibres are strongly developed. The blastostyles may 

 have a mouth. 



Allman believes that the tentacular polyps are only rarely 

 present, and regards them as abnormal modifications of other 

 hydranths. 



Weismann (15), in 1883, described the blastostyles and 

 migration of the sex-cells in Hydractinia. He figures a blasto- 

 style with a mouth and ciliated endoderm, and mentions the 

 occurrence of food granules in the endoderm of the upper 

 region of the body. 



Miss Bunting (17), in 1894, published an account of the 

 origin of the sex-cells and the development of Hydractinia. 

 She first observes the ova in the endoderm of the blastostvle, 

 and concludes that they are probably endodermal in origin. 



My observations on the structure of Hydractinia echi- 

 nata have led me to agree to a certain extent with Strethill- 

 Wright with respect to the relations of the ccenosarc and 

 chitin, and to differ from Allman and Hindis as to their views 

 of the tubular nature of the adult skeleton. 



Moreover I have found a dactylozooid with a mouth, the 

 existence of which has hitherto been overlooked or denied, 

 while other observations show that there is a migration of 

 ova between ectoderm and endoderm in the blastostyle. 



Colonies of Hydractinia are generally situated on the surface 

 of shells, which are commonly whelk-shells inhabited by hermit- 

 crabs. A large colony may cover the whole shell except for 

 a small roundish patch where the shell rubs along the ground 



