718 THE ANIMAL KINGDOM. 



radially from the bases of the four cilia plates in question. They pass 

 through four openings in the bell, divide each into two, and the eight 

 nerves thus formed abut one against the first ctenophoral plate of each of 

 the eight rows. The long axis of the sensory ellipse is prolonged outwards 

 on each side, and in the same plane by the polar plates which in the adult 

 are rectangular in outline. They are ciliated and their margins thickened. 

 The central area of each polar plate leads into the bell by an aperture ; its 

 cilia work towards this aperture and renew the water within the bell. The 

 central nervous system can be retracted by special muscles. 



The funnel extends upwards beneath the central nervous system as the 

 funnel vessel ; it bifurcates and each branch forks again into two ampullae, 

 making four in all, one to each interradius. Two, which correspond to 

 the anterior left interradius and the posterior right, remain blind, whilst the 

 other two are prolonged each into an excretory or anal tube opening 

 externally in the right anterior interradius, and the posterior left just out- 

 side the polar areae. These apertures are usually closed but are opened by 

 the animal for the escape of water and faecal particles 1 . 



The tentacles are two in number, and originate from a thickened basis 

 which is sunk in a deep pit or tentacle sheath, the aperture of which is 

 turned aborally. Consequently, the basis lies more or less close to the 

 principal axis, in Pleurobrachia below, i.e. orally to, the origin of the 

 funnel. The tentacle vessel, which is excessively short, is not prolonged 

 into the axis of the tentacle, but gives origin to two ampullae which lie in 

 its basis. The main axis of the tentacle is beset with a single series of 

 simple branches. The whole structure is eminently contractile and can be 

 completely withdrawn into its sheath. 



The sexual organs are situated at the sides of each meridional vessel, 

 the male on the sides corresponding to the four interradii, the female on 

 those corresponding to the four radii, i. e. on the subtentacular and sub- 

 ventral aspects of the vessels. They take the form of bands coursing along 

 the whole length of each vessel beneath the rows of ctenophoral plates. 

 The genital products pass into the meridional vessels, thence to the funnel, 

 the stomach, and out by the mouth. 



The Cydippidae possess the typical structure above given. The aboral 

 pole of the body may be produced into two or four processes situated in the 

 funnel plane. 



The Lobatae differ from the typical structure in several respects. The 

 funnel axis is very short compared to the stomachal, and the right and left 

 halves of the body are produced on their oral aspects into large spreading 



1 R. Hertwig (op. cit. infra, pp. 6, 7) found in a specimen of the Cydippid Callianira bialata four 

 excretory pores, of which two at opposite angles, i. e. in corresponding interradii, were smaller than 

 the other two. He observed the pores open from time to time ; at the same time a bundle of cilia 

 was protruded. 



