170 PHYLUM C@ELENTERA. 
GENERAL SURVEY OF CQ@ELENTERA 
Before we proceed to the systematic survey, we may contrast the 
essential structural features of the four classes of Ccelentera. 
I. In the Elydrozoa or Hydromeduse there is no inturned ectodermic 
gullet or stomodzum; 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 Scyphomeduse there is an inturned ectodermic gullet or 
stomodzeum ; 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- 
odzeum ; 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 
hase. 
IV. The Ctenophora are very di- 
vergent and apart from the other 
classes, ¢.g. in rarely having any 
stinging cells, and in having a well- 
defined mesoblast, 
SYSTEMATIC SURVEY 
lass I. HyvpRozoa 
Fic. 87.—Diagram of a gymno- c : oa 
blastic Hydroid.—After All- Solitary polyps like Hydra, hydroid 
man. 
a., Stem; 4,, root 5 ¢., gut cavity; @., 
Peete (dark); @., Rarer 
., horny perisarc; g, ra-like 
Unaraon* Chydranths pom the 
same, contracted; 4., hypostome 
bearing mouth; &%., sac-like repro- 
ductive bud (sporosac); ., a 
modified hydranth (blastostyle) 
bearing sporosacs; 2, medusoid 
“* person.” i 
colonies or zoophytes with medusoid 
reproductive buds, medusoids without 
sedentary stages, colonies of modified 
medusoids. 
1. Order Hydromedusze. — Simple 
or colonial forms in which the sexu- 
ally reproductive persons are either 
liberated as free-swimming medusoids 
or are sessile gonophores. 
