mth 



FIG. 89. Convergence toward a globular form in unrelated pelagic 

 organisms : A, Pelagonemertes, a Nemertean worm which floats on the 

 open sea; its congeners are very elongate and "worm-like" and live 

 near the shore ; p, proboscis ; g, proboscis sheath ; c, cerebral ganglia ; 



B, Trochosphcera, a Rotifer, allied to the worms, which swims by means 

 of a circlet of cilia, c 1 , c 2 ; C, a. free-swimming medusa (Bougainvillea) 

 typical of the phylum Ccelenterata ; D, a Ctenophor (Beroe), a member 

 of a group that is but distantly related to the Ccelenterata, with 

 which, however, it is usually included in textbooks ; ot, balancing organ ; 

 tr, funnel of the digestive canal. (A and D, from Sedgwick; B and 



C, from Parker and Haswell.) 



