132 • TEXTBOOK OF ZOOLOGY 



Gonionemus is a small jellyfish form, measuring about a centi- 

 meter across, and is found in the pelagic waters, along our eastern 

 shores. Its shape reminds one somewhat of an umbrella with a 

 fancy fringe but with practically no handle and made of clear cello- 

 phane. The exumhrella is the convex upper, or aboral side while the 

 suhumhrella is the concave, lower, oral side. A short stalklike part, 

 the manuhiHiim hangs down from the center of the subumbrella. At 

 its distal end is the mouth, bordered by four oral lobes. The mouth 

 is the aperture leading into the internal or gastrovascular cavity 

 which has four radial branches or canals. These radial canals join 

 a circumferential or marginal or ring canal. A circular ledge or fold 

 of tissue which extends inward from the margin of the subumbrella 

 and partially encloses this saucer-shaped cavity, is called the velum 

 (craspedon). From a few to more than eighty almost solid tentacles 

 hang down from the margin of the subumbrella. The cell structure 

 of this animal is made up of an outer ectoderm and an inner endo- 

 derm, with a large amount of jellylike mcsoglea between these two 

 genn layers. Wa\y, leaflike folds hanging in the subumbrella and 

 radiating from manubrium to margin are the gonads. A planula-like 

 hydroid form develops from the egg. The animal is able to swim 

 about by drawing water into the partially enclosed cavity of the 

 subumbrella and expelling it through the aperture formed by the 

 velum with enough force to move the animal in the opposite direction. 

 The pressure is developed by contraction of the body. 



Obelia is a marine, colonial type resembling a branched plant in 

 appearance. The individuals are attached to each other in the 

 colony, and it is fastened to a rock or other substratum by a root- 

 like hydrorhiza. They are distributed in the Atlantic Ocean and 

 Gulf of Mexico out to forty fathoms in depth. The colony begins 

 as a single individual which buds, but they do not separate from the 

 preceding or parent generation. This may continue for several gen- 

 erations. From the hydrorhiza there is an upright stem, the hydro- 

 caidus. This stemlike part gives off lateral branches, hydranths; 

 at the end of each is a mouth and tentacles. These are feeding 

 polyps. Also as branches of the stem, there are the hlastostyles which 

 are modified, nonfeeding polyps capable of producing medusae. The 

 medusa is the third type of individual connected with an Obelia 

 colony. The perisarc, which is composed of chitin, covers the colony. 

 In some parts this is ringed, and it expands at the base of the 



