PHYLUM CTENOPHORA 157 



be retracted a tentacle; the tentacles, on the other hand, may be protruded 

 to a considerable distance and trailed behind when the animal is moving. 

 These tentacles bear cells which secrete an adhesive fluid and which are 

 therefore known as glue cells, or colloblasts. At the aboral pole of the 

 body is a collection of sense cells called a statocyst, which is believed to 

 function in the maintenance of equilibrium. 



Other ctenophores differ greatly from the typical form. In some 

 the tentacles are lacking and the body is bell-shaped; others are pear- 

 shaped, and a very different type, known as the Venus' girdle, is elon- 

 gated, flattened, and bandlike, being sometimes over 3 feet long. 



Ctenophores are monecious, the gonads lying on the walls of the 

 meridional canals, which are under the rows of paddle plates, and the 

 sex cells being passed out through the stomodeum. 



185. Advances in Body Plan, — The arrangement of the bands of 

 paddle plates give to ctenophores the appearance of having radial 

 symmetry, but the arrangement of the internal canals and the presence 

 of oppositely placed tentacles suggest bilateral symmetry. This com- 

 bination of the two is by some termed hiradial. Since bilateral sym- 

 metry is usually associated with the possession of a definite anterior and 

 posterior end and a greater degree of purposefulness in movement, the 

 ctenophores represent an advance over the coelenterates. 



Another advance is the development of a mesoderm, simple as it is 

 and represented merely by muscle cells. In contrast to the contractile 

 fibers of coelenterates, which are only parts of cells, these muscle 

 fibers are themselves modified cells. Also what is here termed a meso- 

 derm is formed from the entoderm during the gastrula stage, while the 

 cells which may be found in the mesoglea of coelenterates are derived 

 much later from the ectoderm or entoderm and do not represent a true 

 germ layer. Ctenophores may thus be considered triploblastic. This 

 advance is significant in that it is the first step toward the building up 

 of the powerful bodies possessed by higher forms and composed mainly 

 of tissues derived from the mesoderm. 



186. Activities. — Like the jellyfishes, ctenophores float in the water 

 and are carried here and there by tide and wind. In locomotion the 

 animal is propelled by the paddle plates which strike from the oral, or 

 anterior, end backward. The movement is initiated, however, at the 

 posterior end and wave after wave of movement passes rhythmically 

 from this end forward along each row of these plates. When the animal 

 is seen in the sunlight this frequently gives the effect of successive series 

 of rainbow colors passing from one end of each band to the other. As 

 the animal swims the tentacles trail behind, ready to capture any small 

 organism that may come in contact with them and be held either by 

 adhesion or by the coiling of the tentacles. Though the paddle plates 



