PLEUROBRACHIA. 27 



tural details of those animals, so that in them also breathing and 

 moving are in direct relation to each other. To those unaccus- 

 tomed to the comparison of functions in animals, the use of the 

 word breathing, as applied to the introduction of water into the 

 body, may seem inappropriate, but it is by the absorption of 

 aerated water that these lower animals receive that amount of 

 oxygen into the system, as necessary to the maintenance of life in 

 them, as a greater supply is to the higher animals. The name 

 of Ctenophorae or comb-bearers, is derived from these rows of 

 tiny paddles which have been called combs by some naturalists, 

 because they are set upon horizontal bands of muscles, see Fig. 

 29, reminding one of the base of a comb, while the fringes are 

 compared to its teeth. These flappers add greatly to the beauty 

 of these animals, for a variety of brilliant hues is produced along 

 each row by the decomposition of the rays of light upon them 

 when in motion. They give off all the prismatic colors, and as 

 the combs are exceedingly small, so that at first sight one 

 hardly distinguishes them from the disk itself, the exquisite play 

 of color, rippling in regular lines over the surface of the animal, 

 seems at first to have no external cause. . 



Pleurobrachia. (Pleurobrachia rhododactyla AG.) 



Among the most graceful and attractive of these animals are 

 the Pleurobrachia (Fig. 29), and, though not first in order, we 

 will give it the precedence in our description, because it will 

 serve to illustrate some features of the other two groups. The 

 body of the Pleurobrachia consists of a transparent sphere, vary- 

 ing, however, from the perfect sphere in being somewhat ob- 

 long, and also by a slight compression on two opposite sides 

 (Figs. 27 and 28), so as to render its horizontal diameter longer 

 in one direction than in the other (Fig. 30). 

 This divergence from the globular form, so slight 

 in Pleurobrachia as to be hardly perceptible to 

 the casual observer, establishing two diameters of 

 different lengths at right angles with each other, 

 is equally true of the other genera. It is inter- 

 esting and important, as showing the tendency in 



Fig. 27. Pleurobrachia seen at right angles to the plane in which the tentacles are placed. (Agassiz.) 



