TYPES OF CCELENTERA A SEA- ANEMONE. 153 



Contrast between Medusoids (Hydromedusce) and 

 Medusa 1 . (Scyp homed it see). 



MEUUSOIDS. (CRASPEDOTA.) 



MEDUS.E. (ACRASPEDA.) 



The majority are small " swimming- 

 bells." 



A flap or velum (craspedon) projects in- 

 wards from the margin of the hell. 



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 hydroids, but 

 may have a small sedentary polype 

 stage (or Scyphistoma) in the course 

 of their life history. 



Probably more nearly related to 

 Anthozoa than to Hydromedusae. 



Fourth Type 0/CrELENTERA. 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. Edivardsia and Cerianthus, 

 but all are able to shift their positions by short stages. 

 They feed on small animals - - molluscs, crustaceans, 

 worms, which are caught and stung by the tentacles ; 

 but many depend largely on minute organisms, while 

 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 

 sometimes developed within the parent), some sea-anemones 



