RUDIMENTARY ORGANS OF ANIMALS 



125 



two extreme types come intermediate types which 

 exhibit every possible stage of degeneration. 



In the Ctenophore group development and de- 

 generation are exhibited simultaneously in the 

 organs of locomotion. The fundamental and typical 

 shape of the Ctenophore is round or oval, and the 

 eight sides are provided with swimming plates, 

 originally uniform as in Beroe. 

 The individuals belonging to 

 this group exhibit important 

 evidences of modification in 

 their external morphology. The 

 body being sometimes com- 

 pressed in various directions, 

 the shape is altered from the 

 original, and assumes a more 

 or less irregular appearance. 

 The organs of locomotion 

 undergo a corresponding 

 change. Take for example 

 an adult specimen of Bolina 

 norvegica (fig. 5 3) ; the body is lobate, although it 

 was round during the larval period ; the swimming 

 plates are not uniform, four being long and reaching 

 along the whole length of the body, the other four 

 being only developed in the upper half of the body 

 as far as where the lobes are inserted, where they 

 end as degenerate hair-like processes. By referring 

 to the Cestidse, which are ribbon-like in shape, it 

 will be seen that by means of compression the body 



FIG. 53. Bolina Norvegica, 

 seen from the broad side. 



C, short rows of swimming 

 plates; A, long rows; L, 

 lobes. (After Vogt and 

 Yung, Traite d'anatomie 

 comparte.) 



