STRUCTURE OP ACALEPH^. 129 



shaped disc above contains the stomach, which is placed in 

 the centre, and which opens by a single orifice or mouth, 

 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 supply of 



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. Eecent 

 discoveries in regard to the developmental history of the 

 Medusae 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 



