s KKTI 'i. AIM U.K. 



g 



ful 



loo'ieiil structure, especially in the matterof the relation of the cell layer-. Indeed, the more one 

 studies his masterly work the more profound becomes the conviction that a careful and tlioroiiyh 

 use of M-ood powers of observation and interpretation applied to lisiny r at least fresh material 

 is capable of yielding results that will bear comparison with those attained with the use of the 



most advanced tcchnic. 



The writer does not believe that any Worker has done more to elucidate the entire subject of 



hydroid morphology, at least in the groups investigated by tl Ider Ava--i/. than ha- that writer 



himself. Ilis is the only o-ood description that I have been able to lind of the scrtularian 



hydranth.' The subject of one of his care- 

 studies is the common \, /////,///.? jm- 

 i. and he reports the following points 

 the hydranth: : The eienosaiv of 



the stem consists of two cell layers. In the 



stem this is of uniform thickness to a point 



just below the hydrotheca', where it ex 



pands on two opposite sides. o-i\ ino- "tl 



cylindrical extensions which are uniform in 



diameter and puss through the diaphragm 



at the base of the hydn itheca, traverse the 



lono-ili of the latter, and terminate in a 



simple, short, conical proboscis, around 



which a sino-]e row of slender, taper'ui" 1 ten- 

 tacle-, usually sixteen in number, is dis- p _^-</'' '' '',' ' t^'i 



posed in a uniform series. The extended 



hydranth has no thicker ectoderm than in 



the main stem, but the emloderm is twice 



as thick, although the ectoderm and endo- 



derm are about ei|iial in the stem. The 



ectoderm of the body wall is connected in 



places with the hydrothecul wall by lilm- 



like projections or pseudopodial prolonga- 

 tions. 



So far as the present writei- has been 

 able to ascertain, the hvdranths of all of the 

 Sertularida 1 are very similar to those of 

 >'. /iiiniilii. having a conical or dome-shaped 

 probo-cis and a single whorl of tiliform 

 tentacles. (Flu'. '.'.I It is seldom that the 

 hvdranth can lie studied to advantage in 

 pi-es(>rved specimens, as they are usually 

 ei ther in a state of contraction Or have been 

 macerated or in some way disintegrated 

 iii the preserving fluid. I f care is taken, however, it is possible to preserv e expanded hydranths 

 of shallow-water forms by the methods of killing recently devised. It seems useless to expec< 



that we shall ever be able to -tudy the li\ ino- and expanded hydranth- of deep-water species. 



AS indicated in Part I of this work, :1 there is but little dill'erence between the hydranth- of 

 the I'lumularidie and Sertularida'. the main distinction beino- in a con-trictimi some di-tance 

 below the tentacles of the former which di\ ides the hydranth body ca\ ity into two portions, 

 which, however, communicate broadly. 



'Contributions to the Natural lli-t..ry ..f the Knit.-.! stat.-. IV. ist;-j. pp. 328-329, pi. \\\u. 



The terminology n.-. .1 l.y I'rofei-.-or A-_-:i i/ i- 80 .lil'lVn-nt ti-,,iii that n..\\ in \>.'_'iie that I liave thoiiL'ht it he-t to 

 the follow in-i point- in more modern plnu-.-' !. 'L'\ for the >ake of luci.liu . 



9. 



. '.'. llyilr- \ ] in ii. I. '.T 1 1 \.lra in : 



rlllllln; /.. /'. |ir,itriii-ti'l>. Tin- l"ly i-.ivity i- ill.lic-ati-.l by tin- i 



axial i...rti..ii of tin; liyilrimtli. 



