122 



COLLEGE ZOOLOGY 



Gastral filament £^^^^00 



Subgenital pit 

 Gastrodermis 

 Mesoglea 



Epidermis Gonad 



Gonad 



Radial cana 

 Lappet 



Tentacle 



SEXUAL 

 REPRODUCTION 



Strobila 



medusa (section) 



t/t 



E 



D 



O 



c 

 o 



</> 

 0) 

 O) 



o 



y Blastula 



Gastrula 

 (section) 



Scyphlstoma 



Attached stage 



Planula swims free 

 of oral arm 



ASEXUAL 

 REPRODUCTION 



Figure 60. Life cycle of the jellyfish Aurellia. Longitudinal sections through gastrula stage. 

 Vertical section through adult. 



Aurellia are, in general, similar to those of 

 the hydra. 



The gonads are frill-like organs lying in 

 the floor of the gastric pouches. The egg de- 

 velops into a free-swimming planula which 

 becomes attached to some object, and devel- 

 ops into an elongated and deeply constricted 

 polyp, known as the scyphistoma stage 

 (Fig. 60). The scyphistoma becomes di- 

 vided into disks, resembling a pile of 

 saucers; at this stage it is known as a strobila. 

 Each disk develops tentacles; and, separat- 

 ing itself from those below, it swims away 

 as a small medusa called an ephyra. The 



ephyra gradually develops into an adult 

 jellyfish. 



Metridium— a sea anemone 



A common representative of the class An- 

 thozoa is the sea anemone (see colored 

 frontispiece at beginning of text). Metri- 

 dium dianthus (Fig. 61), is an anemone 

 which fastens itself to the piles of wharves 

 and to solid objects in tide pools along the 

 north Atlantic coast. It is a cylindrical ani- 

 mal with a crown of hollow tentacles, ar- 

 ranged in a number of circlets about the 



