The Coelenterata 255 



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 ( ) which soon 

 acquires a central cavity and becomes fixed to some object. This then 

 forms a new colony. 



When there is an alternation of generations, one sexual, reproducing 

 by eggs and spermatozoa, and the other asexual, reproducing by division 

 or budding, such an 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 sper- 

 matozoa. 



Hydra have no 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 water into the subumbrella-cavity, and then forces it 

 out through the central opening in the velum by contracting its body, 

 thus propelling itself in the opposite direction. This method of locomo- 

 tion is called hydraulic. It is common to all medusae. 



The tentacles (from sixteen to about eighty in number) are capable 

 of great contraction. Adhesive or suctorial pads are found near their 

 tips. Hanging 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 circum- 

 ferential 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 and gives the animal a jelly-like consistency. 

 Then there are many nerve cells scattered about beneath the ectoderm, 

 and a nerve ring is placed about the velum. 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 containing pigmented entoderm cells and 

 communicate with the circumferential canal. (2) Those between the 

 bases of the tentacles are small outgrowths, probably organs of equi- 



