CLASSES OF CCELENTERA 201 



(3) Or the colony may raise itself in the water by forming a common 

 upright coenenchyma, in which the polyps are embedded, and the 

 medullary part of which may form a substantial axis of cemented 

 spicules, e.g. Corallium, a type of Pseudaxonia. 



(4) Or tiie vertical extension of the colony may be effected by a 

 horny secretion from the polyps, which comes to form an axis, really 

 outside of the polyps though encrusted by them. This axis may 

 be purely horny or in part calcareous, e.g. Gorgonia and Acanella, types 

 of Axifera (Fig. 104, III.). 



(5) Fifthly, the vertical extension may be due to a great elongation 

 of a single primary polyp which gives off solenia bearing numerous 

 secondary polyps, e.g. Pennatida, a type of Stelechotokea (cf. Fig. 



104, IV.). 



An altogether aberrant type is represented by the blue coral 

 {Heliopora) and its extinct relatives {Heliolites, etc.). 



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 stomodaBum ; 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 

 frequentlv a combination of polypoid and medusoid phases in the life- 

 history ; the circumference of the medusoid bears a muscular velum of 

 ectoderm and mesogloea ; there is no calcareous secretion (except in 

 Millepores). 



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

 StomodaBum ; 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 intiurned ectodermic gullet or 

 stomodffium ; 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 mesoderm. 



SYSTEMATIC SURVEY 



Class I. Hydrozoa 



Solitary polyps hke Hydra, hydroid colonies or zoophytes with 

 medusoid reproductive buds, medusoids without sedentary stages, 

 colonies of modified medusoids. 



