COELENTERATA 



79 



Fig. 65. Pennatula sulcata. 



of the Anthozoa. The coelenteron is 

 somewhat complex, owing to the fact 

 that there are often more than four 

 radial canals, and that these generally 

 branch greatly as they approach the 

 margin of the bell. 



The sexes are usually separate, and 

 the germ-cells are set free into the coel- 

 enteron, and pass out through the mouth. 

 The fertilized ova may develop directly 

 into new medusae, but in some cases there 

 is a suggestion of an alteration of genera- 

 tions, for the ovum gives rise to a polyp 

 (Fig. 68, E and F) which in general 

 structure resembles the anthozoan polyp. 

 It possesses an oesophagus, and there are 

 four mesenteries in the coelenteron. This 

 polyp is known as a scyphistoma, and. 



Unlike the hydroid polyp, it does not The sea pen; entire colony 



remain long in this Condition SO that it f : lateral branch (After Koel- 

 nker, from Parker and Has- 



must be regarded rather as a larva, for it well's Manual.) 



may develop into a medusa, or by a series 



of transverse constrictions give rise to a series of young medusa , 



called ephyrae (Fig. 68, H and I), attached one above the other, 



the whole structure being called 

 a strobila(Fig. 68, G). As they 

 mature, each ephyra separates 

 from the rest, and develops 

 into an adult medusa. 



Most of the Scyphomedusae 

 are free-swimming, pelagic ani- 

 mals, varying greatly in size 

 from a few centimeters to nearly 

 two meters in diameter ; and 

 many possess on the exumbrella 

 and tentacles a large number of 

 nettle cells, so that the larger 



FIG. 66. A common sea anemone. (After • , ,. i „j.i„ 



Emerton, from Parker and Harwell's animals Can sting unpleasant.} . 



Manual.) The tentacles are often very 



