70 MANUAL OF ANIMAL BIOLOGY 



Suspended from the center of the oral surface is the manubrium, 

 which ends in a wide-lipped mouth. The lips are known as the 

 oral lobes. The manubrium, near its attachment to the oral 

 surface of the body, merges into four radial canals. These run 

 at right angles to each other from the manubrium to the edge of 

 the body near where the tentacles are attached, and there they 

 connect with the circular canal which encircles the body. The 

 enteric cavity, which begins in the manubrium, is continuous 

 throughout the radial and circular canals. The sexual reproductive 

 organs, or gonads, are suspended from the radial canals, and are 

 either male or female. 



Ectoderm. The body wall of the medusa is similar in con- 

 struction to that of all Coelenterates. It consists of an outer layer 

 of ectoderm and an inner layer of endoderm. The two cellular 

 layers are separated by the non-cellular mesogloea. The ectoderm 

 cells form a covering over the entire body, including the oral and 

 aboral surfaces and tentacles. The nematocysts are confined to the 

 tentacles. The tentacles also bear a number of adhesive discs 

 which arise as modifications of the ectoderm cells. 



The ectodermal nerve cells are more highly developed in a jelly- 

 fish than in Hydra, and both sensory and motor nerve cells 

 are present. In the ectoderm near the attachment of the velurn, 

 the nerve cells are grouped to form the nerve rings. The latter 

 encircle the body in the region near the circular canal. The cells 

 of the nerve ring are chiefly motor in function. Two types of spe- 

 cialized sense organs are formed in the ectoderm. At the base 

 of each tentacle there is a spherical cavity containing pigmented 

 ectodermal cells, while between the bases of the tentacles appear 

 small, solid, ectodermal outgrowths, known as statocysts, which 

 are believed to function as organs of equilibration. The periph- 

 eral sense organs are able to receive certain stimuli from the 

 environment and pass an impulse on to the inner nerve cells. 

 This is exactly what the sense organs do in the higher animals. 



The stimuli received by the sensory cells are also able to bring 

 about a high type of coordinated swimming movement. In swim- 

 ming, there is a rhythmic contraction of the entire body brought 

 about by the muscular elements present in the cells of both the 

 ectoderm and the endoderm, under the control of the motor nerve 

 cells. The contraction results in a decrease in the volume of the 

 space between the velum and the top of the oral surface. A 



