iv PHYLUM OCELENTERATA 223 



but, in the absence of meridional canals, situated in rounded outgrowths of 

 the per-radial or transverse canal. The inter-radial canals give off lateral 

 branching, but not anastomosing, canals. 



Tjalfiella differs from all the rest of the Ctenophora in being viviparous. 

 The young, escaping by rupture of the body-wall of the parent from the 

 brood-pouches (apparently in the canal-system) in which they are developed, 

 are provided each with eight swimming-plates and pursue a free existence for 

 a time, subsequently becoming attached and losing the swimming-plates. 



Gastrodes, a parasite of Salpa (Vol. II.), is probably related to Ctenoplana 

 and Cceloplana. 



[Bathyctena, also a deep-sea form, with some resemblances to Tjalfiella, 

 is more nearly related to the normal Cydippida.] 



The Platyctenea are of interest owing to the possibility of their representing 

 an intermediate stage between Ccelenterates (through ordinary Ctenophores) 

 and Platyhelminthes (see Section V.). 



The Ctenophora are usually perfectly transparent, and quite 

 colourless, save for delicate tints of red, brown, or yellow in the 

 tentacles and stomodseal ridges. Cestus has, however, a delicate 

 violet hue, and when irritated shows a beautiful blue or bluish- 

 green fluorescence. Beroe is coloured rose-pink. 



Ctenophora are found in all seas from the arctic regions to the 

 tropics. As is to be expected from their perishable nature, there is 

 no trace of the group in the fossil state. 



A very remarkable fact has been made out with regard to Bolina 

 liydatina, one of the Lobata, a Ctenophore which attains a diameter 

 of 25-40 mm. While still in the larval or cydippid condition and 

 not more than 0-5-2 mm. in diameter, it becomes sexually mature, 

 the gonads producing ripe ova and sperms ; and the eggs are im- 

 pregnated and develop in the usual manner. Soon the gonads de- 

 generate, the larva metamorphoses into the adult form, and a second 

 period of sexual maturity supervenes. This precocious ripening of 

 sex-cells occurs as we shall see in other animal groups, and is 

 called pcedogenesis (p. 42). 



THE RELATIONSHIPS OF THE CCELENTERATA. 



There can be little doubt that the lowest ccelenterate form known 

 to us is the simple hydrozoan polype, represented by Hydra and 

 by the hydrula stage of many Hydrozoa. Somewhat more complex 

 in virtue of its stomodseum (if a true stomodseum be indeed repre- 

 sented) and its gastric ridges and filaments is the scyphozoan 

 polype, represented by the scyphula of Aurelia. Still more 

 complex is the actinozoan polype, or actinula, as it may be called, 

 with its large stomodseum, mesenteries and mesenteric filaments, 

 and elaborate muscular system. Speaking generally, one may 

 say that these three polype-forms represent as many grades of 

 organisation along a single line of descent. 



The medusa-form in the Hydrozoa is, as we have seen, readily 

 derived from the hydrula by the widening out of the tentacular 

 region into an umbrella. We may thus conceive of the Trachylinse, 



