254 GENERAL BIOLOGY 



break out into the water. The eggs are fertilized by spermatozoa which 

 have escaped from other medusae. Cleavage is similar to that of Hydra, 

 and a hollow blastula and solid gastrula-like structure are formed. The 

 gastrula-like structure soon becomes ciliated and elongates into a free- 

 swimming larva called a planula ( ). This soon acquires a 

 central cavity, becomes fixed to some object, and proceeds to found a 

 new colony." 



When there is an alternation of generations, the one being sexual, 

 reproducing by eggs and spermatozoa, and the other asexual, reproduc- 

 ing by division or budding, such as alternation of generations, is called 

 metagenesis ( ). 



In Obelia the -asexual generation (the colony of polyps) forms buds 

 of two kinds, the hydranths and the gonangia ( ). The 



sexual generation (the medusoid) reproduces only by eggs and spermat- 

 ozoa. 



Hydra do not have a regular medusoid stage and Geryonia (Fig. 

 157A), ( ), no polyp or hydroid stage. 



Gonionemus (Fig. 157B), ( ) 



The structure of a medusa or hydrozoan jellyfish is well illustrated 

 by Gonionemus, which is quite common along the eastern coast of the 

 United States. It is about half an inch in diameter. In general form 

 it is similar to the medusa of Obelia. The convex or aboral surface is 

 called the exumbrella ( ), and the concave, or oral 



surface, the subumbrella ( ). The subumbrella is 



partly closed by a perforated membrane called the velum ( ). 



The animal takes in water into the subumbrella-cavity, and then forces 

 it out through the central opening in the velum by the contracting of 

 its body, thus propelling the animal in the opposite direction. This 

 method of locomotion is called hydraulic. It is common to all medusae. 

 The tentacles (from sixteen to about eighty) are capable of great 

 contraction. Adhesive or suctorial pads are found near their tips. Hang- 

 ing down into the subumbrellar cavity is the manubrium with the mouth 

 at its end surrounded 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 structure of Gonionemus is similar to that of Hydra, 

 but the mesoglea is thicker which gives the animal a jelly-like consist- 

 ency. Then there are many nerve cells scattered about beneath the 

 ectoderm, and a nerve ring is placed about the velum and there are 

 sensory cells with a tactile function on the tentacles. At the margin of 

 the umbrella there are two kinds of sense organs: (1) Those at the base 

 of the tentacles are round bodies which contain pigmented entoderm 

 cells and communicate with the circumferential canal ; (2) those between 

 the bases of the tentacles, Avhich are small outgrowths, probably organs 



