1 62 



PHYLUM CCELENTRRA. 



cndodermic muscles circular ; the sex cells are usually produced in the 

 ectoderm ; there is very frequently a combination of polypoid and 

 medusoid phases in the life history ; the circumference of the medusoid 

 bears a muscular velum of ectoderm and mesoderrri ; there is no cal- 

 careous secretion (except in Millepores). 



II. In the Scyphomedusce there is an inturned ectodermic gullet or 

 stomodaeum ; there are hints of mesenteries ; there are special digestive 

 filaments ; the sex cells are endodermic ; there is no velum ; there is 

 often a non-sexual sedentary stage ; there is no calcareous secretion. 



III. In the Anthozoa there is an inturned ectodermic gullet or 

 stomodreum ; there are distinct mesenteries or partitions from body 

 wall to gullet wall ; there are often digestive filaments ; in the body wall 

 the ectodermic muscles are circular (except in Cerianthus], and the 

 endodermic muscles longitudinal ; the sex cells are endodermic ; there 

 is no medusoid phase. 



IV. The Ctenophora are very divergent and apart from the other 

 classes, e.g. in rarely having any stinging cells, and in having a well- 

 defined mesoblast. 



SYSTEMATIC SURVEY. 

 Class I. HYDROMEDUS^E. 



Solitary polypes like Hydra, hy- 

 droid colonies or zoophytes with 

 medusoid reproductive buds, medu- 

 soids without sedentary stages, 

 colonies of modified medusoids. 



I. Order Hydromedusae. Simple 

 or colonial forms in which the sexually 

 reproductive persons are either liber- 

 ated as free-swimming medusoids or 

 are sessile gonophores. 



(a) Hydrophora. Two types are 

 included here. The first includes the 

 Tubularians, Hydractinia^ and other 

 forms in which the polypes are not 

 enclosed in the protective sheath 

 which often surrounds the colony 

 (gymnoblastic), and in which the free 

 medusoid forms, when present, have 

 their genital organs placed in the wall 

 of the manubrium (Anthomedusce), 

 and are furnished with ocelli placed at 

 the base of the tentacles. Hydra and 

 its allies may be included here. 



An unattached marine hydroid 

 Hippolytus peregrines has been de- 

 scribed, and as it bore gonophores it 

 was obviously mature, which is doubt- 

 ful as regards two other unattached 



FIG. 77. Diagram of a gymno- 

 blastic Hydromedusa. After 

 Allman. 



a., Stem ; b., root ; c., gut cavity ; d., 

 endoderm (dark); e. , ectoderm; 

 _/I, horny perisarc : "., hydra-like 

 "person" (hydranth); g?., the 

 same, contracted ; //., hypostome 

 bearing mouth ; /., sac-like repro- 

 ductive bud (sporosac) ; m., a 

 modified hydranth (blastostyle) 

 bearing sporosacs ; /. , medusoid 

 " person." 



