144 



CCELENTERA. 



exceptionally in Aurelia. Cyanea is often very large, "it may measure 

 7| ft. across the bell, with tentacles 120 ft. long." Chrysaora is her- 

 maphrodite, and has diffuse sperm sacs even upon the arms. In the 



Rhizostomze, e.g. Cassiopeia and 





Pi/ema, the mouth is obliterated, 

 and replaced by numerous small 

 pores on the four double arms. 

 Litcemaria and its allies are 

 interesting sessile forms which 

 have been compared to sexual 

 Scyphistomas, that is, are re- 

 garded as persistently larval 

 forms. 



We may note here that Chun, 

 while agreeing provisionally to 

 the separation of the Acraspeda 

 from the Hydrozoa, strongly 

 opposes their association with the 

 Anthozoa, basing his opposition 



especially on the existence of Scyphistomas of great simplicity {e.g. 



Spongicold). 



Fig. 62 



—I.Hicrnarla. — After 

 Korotneff. 



Contrast between Hydrozoon Medusoids a.7id Scyphozoon 

 MeduscB. 



Hydrozoon. (Craspedota.) 



The majority are small "swimming 

 bells." 



A flap or velum (craspedon) projects in- 

 wards from the margin of the bell. 



No tainiolae, nor gastric filaments. 



A double nerve-ring around the margin. 



Naked sense organs either optic or audi- 

 tory. Theyare usually derived from 

 the skin, but the auditory sacs may 

 be modified tentacles. 



Reproductiveorgans 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- 

 medusse, all arise as the liberated 

 reproductive persons of Hydroid 

 colonies. 



True Hydrozoa. 



SCVPHOZOON. (ACKASl'EDA.) 



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 

 tasniolse, from part of which the 

 gastric filaments of the adult grow. 



Eigbt 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 Hydrozoa. 



