238 



CLASSIFICATION OF ANIMALS 



y^r 



p^'"'-'-^l^ 



(tentacles, eyespots, statocysts) and nerve centres alternating 

 with the lobes. There is no velum. The margin of the peri- 

 stome is four cornered with the four corners prolonged into ''oral 



lobes" or branching tentacular arms. 

 There are four groups of gastric fila- 

 ments projecting into the central gastric 

 pouch and the radial canals branch 

 many times. The gonads are four in 

 number and are developed from the 

 entoderm of the gastric cavity into 

 which they project. The embryo is set 

 free as a ciliated larva. It swims about 

 for a time then becomes attached and 

 develops into a hydra-like polyp. This 

 polyp is called a scyphostoma and from 

 it many medusae are formed by strobil- 

 lation. The polyp becomes constricted 

 just below the circle of tentacles and 

 another circle of tentacles begins to 

 form below this constriction. Then a 

 third circle of tentacles and a third con- 

 striction begin. This process continues 

 downward while the polyp grows in 

 length. The uppermost constriction 

 grows continually deeper until a disc- 

 shaped portion of the parent polyp 

 with a margin of tentacles is completely 

 cut off. There is thus formed a minute 

 medusa which is called an ephyra. 

 Later another disc is cut off and then another. By this method 

 a number of ephyrae are produced and they develop directly 

 into medusae. Sometimes the scyphostoma stage is omitted, 

 the ciliated planula larva developing directly into the 

 medusa. 



Fig. 134. — Physalia, the 

 Portuguese man-of-war. 

 (From Galloway, after 

 Agassiz.) 



