CTENOPHORA. 199 



plates the ribs. These rows begin close to the aboral pole, and 

 pass in the meridians of the animal towards the oral pole. Four 

 of these rows lie in one half of the body, and four in the '<>ther. 

 Further it is to be noted, that of these eight rows we may distinguish 

 those which lie on each side of the tentacle (or funnel-plane) these 

 are the sub-tentacular rows*' and those which lie on each side of 

 the stomach-plane, which we may call the sub-stomachal f rows 

 (Fig. 158). There are therefore four sub-tentacular rows and four 

 sub-stomachal rows of meridional plates. 



The central nervous system and sense organ are placed at the aboral 

 pole. (Fig. 159). It has the form of a flat depression formed of 

 ciliated sensory ectoderm, and covered over by a bell-shaped structure 

 (the bell) formed of fused cilia (gl). Some of the cilia of the sensory 

 area are very long and fused together to form four large triangular 

 plates. These are the springs (/). Their tips are attached to, and 

 carry a small mass of otoliths (ot), which is placed over the centre 

 of the sensory area. The sensory plate is drawn out in the stomach 

 plane into two lobes the polar plates (Fig. 158, j). Beginning at 

 the base of each of the four springs, or otolith-bearers, is a ciliated 

 groove (pi), which, passing outward through a hole in the bell-like 

 cover, divides into two grooves (ft 1 " 8 ). These are continuous with the 

 aboral ends of two rows of vibratile plates, and are sometimes called 

 nerves because they seem to transmit any movement of the otolith- 

 bearer to the row of vibratile plates, and so set the latter in motion. 

 The movement of the vibratile plates begins at the aboral pole and 

 passes oralwards, each plate successively bending energetically 

 towards the aboral pole, and then slowly regaining its original posi- 

 tion. The movement of the animal is thus with the oral end 

 forwards. 



The vibratile plates have the appearance of consisting of long cilia 

 fused together at their bases. They arise from specially long ectoderm 

 cells. 



The gastro vascular apparatus (Fig. 158) consists of a central space, 

 the funnel, which gives off two vessels the perracUal vessels (c.pr); 

 these pass outwards in the funnel-plane in opposite directions and 

 divide dichotomously into the interradial vessels (c.ir), of which 

 there are four. These again divide, and give rise to eight adradial 

 vessels (c.adr\ which enter the meridional vessels. The meridional 

 vessels underlie the rows of vibratile plates and end blindly above 



* Sometimes called sub-transversal. 



t Sometimes called sub-sagittal and sub-ventral. 



