PHYLUM C(ELENTERATA 



123 



the exumbrella; the concave, or oral surface, the subumbrella. 

 The subumbrella is partly closed by a perforated membrane 

 called the velum. Water is taken into the subumbrellar cavity 

 and is then forced out through the central opening in the velum 

 by the contraction of the body; this propels the animal in the 

 opposite direction, thus enabling it to swim about. 



The tentacles, which vary in number from sixteen to more than 

 eighty, are capable of considerable contraction. Near their tips 

 are adhesive or suctorial 

 pads at a point where the 

 tentacle bends at a sharp 

 angle. Hanging down into 

 the subumbrellar cavity is 

 the manubrium with the 

 mouth at the end sur- 

 rounded by four frilled 

 oral lobes. The mouth 

 opens into a gastrovascular 

 cavity which consists of a 

 central " stomach " and four radial canals. The radial canals enter 

 a circumferential canal which lies near the margin of the umbrella. 



The cellular layers in Gonionemus are similar to those in Hydra, 

 but the mesoglea is extremely thick and gives the animal a jelly- 

 like consistency. Scattered about beneath the ectoderm are 

 many nerve cells, and about the velum is a nerve ring. Sensory 

 cells with a tactile function are abundant on the tentacles. The 

 margin of the umbrella is supplied with two kinds of sense organs : 

 (1) at the base of the tentacles are round bodies which contain 

 pigmented entoderm cells and communicate with the circumfer- 

 ential canal; (2) between the bases of the tentacles are small out- 

 growths which are probably organs of equilibrium and, therefore, 

 statocysts. Muscle fibers, both exumbrella and subumbrella, 

 are present, giving the animal the power of locomotion. 



Suspended beneath the radial canals are the sinuously folded 

 reproductive organs or gonads. Gonionemus is dioecious, each 



Fig. 74. — Gonionemus, a hydrozoan jelly- 

 fish. (From Washburn, after Hargitt.) 



