CCELENTERA TA. 135 



The HydrocorallincB are peculiar in having a massive 

 calcareous skeleton instead of the usual chitinous one and 

 for their very primitive little medusoids. Their calcareous 

 skeletons can be distinguished from the true corals by the 

 absence of septa in the apertures left by the polypes. The 

 Siphonophora are floating pelagic colonies with little or no 

 skeleton but with remarkable division of labour, the members 

 of the colony being modified into a great variety of kinds. 



Class II. — Scyphozoa. (Actinia and Aurelia.) 



In these the coelenteron at one time of their life is 

 divided into central (gastric) and peripheral (vascular) 

 cavities, and there is usually an ectodermal gullet. The 

 gastric cavity usually has gastric filaments and the gonads 

 are endodermal. Aurelia represents those types which have 

 hydroid and medusoid phases, but a number of other jelly- 

 fishes have only the medusoid phase. All these form the 

 sub-class Scyphomedusce. The important forms with only 

 hydroid phase {e.g., Actinia) form the sub-class Actinozoa. 

 Actifiia, like Hydra, is solitary and without an exoskeleton, 

 but actinozoan colonies (like hydroid zoophytes) also occur. 

 The skeleton, usually ectodermal, is most commonly of 

 calcareous matter, and may assume vast proportions. 

 These colonial types are called corals and their skeletons 

 may be recognised by the presence of radial septa in the 

 holes formerly inhabited by the polypes. (Coral Islands, 

 see page 72.) The Actinozoa are divided into two important 

 orders, the Hexactinia and Octactinia, according to the 

 number of mesenteries and other structural features 

 mentioned in the types Actinia and Alcyonium. 



Class III. — Ctenophora. 



The unique motor organs of this class tend to separate 

 them from the other two classes, but they are connected by 

 certain intermediate forms. 



Cydippe is a very fair representative of the class. 



They are typically free-swimming pelagic organisms of 

 carnivorous habits. Some {Cestum) become elongated in 

 one plane to form a long ribbon, or they may (Bero'e) form 

 a large bell by increase of the stomod^um. 



