CCELENTERA, 



compared with Ilydrozoa, 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 

 Aitrelia^ some, e.g., Felagia, have 

 a direct development without the 

 intervention of Scyphistoma or 

 Strobila stages, but this may 

 occur exceptionally in Aiirelia. 

 Cyanea is often very large, " it 

 may measure y-^- feet across the 

 bell, with tentacles 120 feet 

 long." Chrysaora is hermaphro- 

 dite, and has diffuse sperm sacs 

 even upon the arms. In the 

 Rhizostomce — e-g--, Cassiopeia and 

 Pilema^ the mouth is obliterated, 

 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. 



Cojitrast between Hydi-ozoon a?id Scyphozoon medusoids. 



Fig. 45. — Lucernaria. {After 



KOROTN'EFF.) 



Hydrozoon (Craspedota.) 



small 



swimming 



The majority are 



bells." 

 A flap or velum (craspedon) projects i 



wards from the margin of the bell. 



No tEcniolEe, nor gastric filaments. 



A double nerve ring around the margin. 



Naked sense organs either optic or audi- 

 tor>'. 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 Trachymedusse, 

 all arise as the liberated reproductive 

 persons of Hydroid coloTiies. 



True Hydrozoa. 



SCVl'HOZOON. (ACRASPEDA.) 



Many are large "jelly fish." 



No velum. (The vclarimn of Aurclia 

 is a mere fringe, \-eiy inconspicuous 

 in the adult, and not inturned.) 



In the Scyphistoma there are four tasniolje, 

 from pari 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. 



Repi"oducti\'e organs in special pockets 

 on the floor of the gastric cavity. 

 I'he 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 Hydro:^oa. 



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 [c.g.^ Spongicola). 



