CONTRAST: CRASPEDOTA AND ACRASPEDA. 137 



classed in the order Discomedusge of the sub-class Scypho- 

 medusse or Acraspeda. Among its neighbours are the 

 following : — Cyanea^ some species of which are very large, 

 e,g,, "an old example of Cyanea arctica which measured 

 iYt. feet across the bell, with tentacles 120 feet long;" Chry- 

 saora^ a hermaphrodite, with diffuse sperm-sacs even upon 

 the arms ; Pelagia, with a direct development, i.e., without 

 Scyphistoma or Strobila stages ; and the Rhizostomae, e.g., 

 Cassiopeia and Pilema, in which the mouth is obliterated 

 and replaced by numerous pores on the four double arms. 



Contrast bePiveen a Hydrozoon and a Scyphozoon medusoid, 

 between a Craspedote and an Acraspedote, 



Hydkozoon. (Craspedota.) 



The majority are small " swimming 



bells." 

 A flap or velum (craspedon) projects 



inwards from the margin of the bell. 



No tceniolae, nor gastric filaments. 



A double nerve-ring around the margin. 



Unhooded sense-organs either optic or 

 auditory. They are usually derived 

 from the skin, but the auditory sacs 

 may be modified tentacles. 



Keproductive 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- 

 raedusae, all arise as the liberated 

 reproductive persons of Hydroid 

 colonies. 



True Hydrozoa. 



Scyphozoon. (Acraspeda.) 



Many are large "jellyfish." 



No velum. (The velaritivi of Aurelia 



is a mere fringe, very inconspicuous 



in the adult, nor inturned.) 

 In the Scyphistoma there are four 



tseniolse, from part of which the 



gastric filaments of the adult g^ow. 

 Eight separate nervous centres beside 



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 reproducive 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. 



Class Scyphozoa. Sub-Class, Anthozoa or Actinozoa. 

 Type. — A Sea-Anemone, such as Tealia crassicornis. 



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

 water mark. Some, e.g.^ Tealia crassicornis^ are often half- 

 buried in sand; a few are unattached. The sedentary 



