68 



THE SCYPHOMEDUSAE 



structure the " hydroid " differs considerably from that of the 

 Hydromedusae, and is for distinction termed the Scyphistoma 

 (scyphula). 



STRUCTURE or THE SCYPHISTOMA. The gastrula, formed by 

 invagination from the blastula, having been converted into a closed 

 sac by coalescence of the lips of the blastopore, affixes itself to a 

 solid object, and a mouth is formed by an ingrowth of ectoderm 

 or stomodaeum, which perforates to the endodermal coelenteron. 

 The appearance of four perradial tentacles is followed by the forma- 

 tion of four interradial, and these by eight adradial tentacles ; all 

 sixteen tentacles are solid. In some cases more than sixteen are 

 developed. To this fixed tentaculate organism is applied the name 

 Scyphistoma (Figs. 6, A, It, C, D ; 7). In some cases it secretes 



a perisarc (Chrysaora). 



Internally the organism presents 

 considerably greater complexity of 

 structure than the hydroid type of 

 Hydromedusae, being built essenti- 

 ally on the same plan as Haliclystus 

 among Stauromedusae. It has 

 four interradial mesenteries (taeni- 

 olae), which have a free edge pro- 

 jecting into the gastric cavity below, 

 but are attached in the oral region 

 to the stomodaeum and subumbrella; 

 they are invaded for a short distance 

 by ectodermal subumbral funnels, the 

 muscle cells of which run deep into 

 the mesogloea. Pliacellae or gastric 

 filaments are not developed, but the 

 thickened edge of the mesentery is 

 probably digestive in function, as in 

 the Anthomedusae. 



The Scyphistoma multiplies (a) 

 by stolonar gemmation from creep- 

 ing horizontal stolons ; (b) by lateral 

 gemmation, the buds, which are 

 pushed out horizontally, bending 

 vertically downwards, becoming 



afrflpliprl tn a tsnlirl nViippf nnrl r\e> 

 a acn ODJCCt, an< 



tached f rom the parent : ( c) by 



, ., . 

 StrODllatlOn. 



STROBILATION AND GROWTH OF 

 THE EPHYRA. The process of strobilation is apparently seasonal. 

 A series of transverse circular furrows constrict the upper or oral 

 part of the Scyphistoma (Fig. 7). In the uppermost of the seg- 



3 



Fio. 7. 



Strobilating scyphistoma of Aurelia 

 (from Shipley, after Haeckel). 1, uncon- 

 stricted base of scyphistoma ; 2, site of 

 tentaculocyst ; 3, adradial tentacle ; 4, 

 marginal guard lappet of future tentaculo- 

 cyst. 



