STRUCTURE OF ACALEPHiE. 
129 
shaped disc above contains the stomach, which is placed in 
the centre, and which opens by a single orifice or month, 
directed downwards. Around the stomach are four chambers, 
in which the eggs are prepared. The mouth is surrounded by 
four large tentacula, which bring to it the necessary 
Fig. 70.— Pelagia. 
food; and other tentacula are seen, in this species, to be 
hanging from the edge of the disc. In the edge of this disc, 
the nutritious fluid, which flows in channels prolonged from 
the stomach and excavated out of the soft tissues, seems to be 
exposed to the influence of the surrounding water; but 
nothing like a heart or a regular circulation exists.—Recent 
discoveries in regard to the developmental history of the 
Medusce and their allies, have rendered it very doubtful 
whether the Acalephce should continue to take rank as a dis¬ 
tinct class; since many of them constitute only a particular 
phase in the life of the Hydroid Zoophytes (§ 124). 
121. The class of Polypifera, or coral-forming animals, 
commonly known as Zoophytes, includes two principal tribes, 
which differ from one another in structure to such a degree as to 
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