TYPES OF CCELENTERA A SEA-ANEMONE. 159 



Contrast between Medusoids (Hydromedusa) and 

 Meduscz (Scyphomedusce) 



MEDUSOIDS. (CRASPEDOTA.) 



MEDUSAE. (ACRASPEDA.) 



The majority are small "swimming- 

 bells." 



A flap or velum (craspedon) projects in- 

 wards from the margin of the bell. 



No taeniolae, nor gastric filaments. 



A double nerve-ring around the margin. 



Naked sense organs either optic or audi- 

 tory. They are usually derived 

 from the skin, but the auditory sacs 

 may be modified tentacles. 



Reproductive organs on the radial canals 

 or by the side of the manubrium. 

 The reproductive cells are usually 

 derived from the ectoderm. 



With the exception of the Trachy- 

 medusae, all arise as the liberated 

 reproductive persons of hydroid 

 colonies. 



Many are large "jelly-fish." 



No velum. (The velarium of Aurelia 

 is a mere fringe, very inconspicuous 

 in the adult, and not inturned.) 



In the Scyphistoma there are four 

 taeniolae, from part of which the 

 gastric filaments of the adult grow. 



Eight separate nervous centres be- 

 side the sense organs, and a sub- 

 umbrellar nervous plexus. 



Sense organs are modified tentacles, 

 and probably have almost always 

 a triple function. They are usually 

 protected by a hood. 



Reproductive organs in special pockets 

 on the floor of the gastric cavity. 

 The reproductive cells arise in the 

 endoderm. 



Have no connection with hyclroids, but 

 may have a small sedentary polyp 

 stage (or Scyphistoma) in the course 

 of their life history. 



Probably more nearly related to 

 Anthozoa than to Hydrozoa. 



Fourth Type #/" CCELENTERA. A Sea- Anemone, such as 

 Tealia crassicornis. Class ANTHOZOA 



Most sea-anemones live fixed to the rocks about low- 

 water mark. All these fixed forms have a distinct basal 

 disc, and .may, like Tealia crassicornis^ be half buried in 

 sand and gravel; others, without a basal disc, are loosely 

 inserted in the sand, e.g. Edwardsia and Cerianthus. All 

 are able to shift their positions by short stages. Some 

 reef-anemones (Cradactts) can crawl about on their 

 tentacles. They feed on small animals molluscs, 

 crustaceans, worms which are caught and stung by the 

 tentacles. Many depend on minute organisms ; others 

 may be seen trying to engulf molluscs decidedly too 

 large for them. A few anemones, without pigment or with 

 little, have symbiotic Algae in their endoderm cells ; the 

 bright pigments of many others seem to help in respiration. 

 Besides the sexual reproduction (in which the young are 



