122 



INVEETEBRATE MORPHOLOGY. 



of the cavity is formed of high ciliated cells probabl} r nervous 

 in function, and above them is a mass of otoliths supported 

 on four incurved rods of fused cilia, one of which forms the 

 termination of each of the four meridional grooves. 



In addition to this sense-organ, which is to be regarded 

 as of the same function as the otocysts of the medusae, there 



csr 



re 



FIG. 67. Pleurobraclda SEEN FOR THE ABOKAL POLE (after AGASSIZ). 

 eg ciliated groove. p = polar urea. 



ot = otocysl. re = radial caual. 



t = tentacle. 



lies at each end of the sagittal axis of the sensory dome a so- 

 called pole-area (Fig. 67, p), the cells of which are furnished 

 with small plates of fused cilia, each area being surrounded 

 by a thickened ciliated rim. These structures are from their 

 form and situation supposed to be sensory, and an olfactory 

 function has been attributed to them. 



The tentacles (Fig. 67, t), of which there are two, situated 

 at the extremities of the transverse axis, are present in all 

 forms except the Beroids. Each tentacle lies in a deep 

 depression termed the tentacle-sheath and consists of a princi- 

 pal axis which gives rise to a large number of secondary 

 tentacles arranged upon one side only. Both the primary 

 axis and the secondary tentacles are solid, being composed 

 mainly of muscle-cells and containing no prolongation of the 

 tentacular vessel. In Mnemiopsis and its allies and in Cesium 



