170 



PHYLUM CCELENTERA. 



GENERAL SURVEY OF CCELENTERA 



Before we proceed to the systematic survey, we may contrast the 

 essential structural features of the four classes of Ccelentera. 



I. In the Hydrozoa or Hydromedusae there is no inturned ectodermic 

 gullet or stomodaeum ; there are no partitions or mesenteries ; there 

 are no special digestive organs ; in the body wall the ectodermic 

 muscles are mostly longitudinal and the endodermic 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 mesoglcea ; there is no calcareous secretion (except in 

 Millepores). 



II. In the Scyphomedusae there is an inturned ectodermic gullet 01 

 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 stom- 

 odaeum ; there are distinct mesenteries 

 or partitions from body wall to gullet 

 wall ; there are often digestive fila- 

 ments ; in the body wall the ecto- 

 dermic 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 di- 

 vergent and apart from the other 

 classes, e.g. in rarely having any 

 stinging cells, and in having a well- 

 defined mesoblast. 



FIG. 87. Diagram of a gymno- 

 blastic Hydroid. After All- 

 man. 



., Stem ; ., root ; c. t gut cavity ; d., 

 endoderm (dark) ; <?., ectoderm ; 

 f., horny perisarc ; , hydra -like 

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

 same, contracted ; /z., hypostome 

 bearing mouth ; ., sac-like repro- 

 ductive bud (sporosac); m., a 

 modified hydranth (blastostyle) 

 bearing sporosacs ; /., medusoid 

 " person." 



SYSTEMATIC SURVEY 

 Class I. HYDROZOA 



Solitary polyps like Hydra, hydroic 

 colonies or zoophytes with medusoic 

 reproductive buds, medusoids withou 

 sedentary stages, colonies of modifiec 

 medusoids. 



I. Order Hydromedusse. Simple 

 or colonial forms in which the sexu- 

 ally reproductive persons are either 

 liberated as free-swimming medusoids 

 or are sessile gonophores. 



