200 



THE ANIMAL KINGDOM 





lie 





Figure 10.19. Pleurobrachia. A, Lateral view of whole animal. B, Detail showing 

 two combs, each formed by the fusion of a row of cilia. (After Hynian.) 



the sea gooseberry, genus Pleurobrachia, and the sea wahiut, genus 

 Mnemiopsis (Fig. 10.1). The mouth is at one end, so that oral, aboral 

 and lateral surfaces can be identified. Each ctenophore swims with eight 

 columns of combs that radiate from the center of the aboral surface 

 over the sides to the oral surface. Each comb is a row of fused cilia (Fig. 

 10.19). Just beneath the epidermis along each comb column is a tract 

 of nerve fibers that coordinate the beating of the cilia. In the resting 

 position each comb points toward the oral end. When the comb bends 

 vigorously toward the aboral end the comb jelly moves through the 

 water mouth first. The combs beat in waves passing along the columns 

 from aboral to oral ends. Synchronized action of the eight comb columns 

 produces a smooth gliding locomotion that may be as fast as two feet 

 per minute. Ctenophores usually swim up and down through a few 

 feet of water with the mouth always forward. 



A comb jelly has but two tentacles. These are branched and can be 

 retracted into tentacle sheaths. Each tentacle has an outer layer of epi- 

 dermis surrounding a core of mesoglea. In the epidermis are numerous 

 colloblasts, each one a modified epidermal cell containing a peripheral 

 hemisphere of adhesive mucus and a basal coiled spring. The spring 

 ejects the mucus against prey and anchors it to the tentacle. 



