148 



CCELENTERA. 



compared with Hydrozoa, in the possession of an ectodermal gullet (see 

 footnote on page 146), but differ in possessing ectodermal septal muscles 

 and in some histological features. 

 Among those closely allied to 

 Aurelia, some, e.g., Pelagia, have 

 a direct development without the 

 intervention of Scyphistoma or 

 Strobila stages, but this may 

 occur exceptionally in Aurelia. 

 Cyanea is often very large, "it 

 may measure "]\ feet across the 

 bell, with tentacles 120 feet 

 long." Chrysaora is hermaphro- 

 dite, and has diffuse sperm sacs 

 even upon the arms. In the 



Rhizostomse e.g., Cassiopeia*.^ p IGp 4 5._Lucernaria. (After 



Pilema, the mouth is obliterated, KOROTNEFF.) 



and replaced by numerous small 



pores on the four double arms. Lucernaria and its allies are interesting 

 sessile forms which have been compared to sexual Scyphistomas that 

 is, are regarded as persistently larval forms. 



Contrast between Hydrozoon and Scyphozoon medusoids. 



HYDROZOON. (CRASPEDOTA.) 



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 Trachymedusae, 

 all arise as the liberated reproductive 

 persons of Hydroid colonies. 



True Hydrozoa. 



SCYPHOZOON. (ACRASPEDA.) 



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 fila- 

 ments of the adult grow. 



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 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. 



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]. 



