IV 



PHYLUM CCELENTERATA 



211 



which, in the present case, are continued the whole length of the oral edge. 

 The young of Cestus (B) resembles a compress 3d Cydippid which undergoes 

 gradual elongation in the median plane. 



-Beroe, the principal genus of the Beroida, has the form of a cylinder (Fig. 158), 

 one end of which is rounded and bears the sense-organ, the other truncated 

 and occupied entirely by the immense mouth (mth.). The greater part of the 

 body is taken up by the huge gullet ; the infundibulum (inf.), per- radial and 

 infundibular canals, &c. , being all crowded into 

 a small space at the aboral pole. The meri- 

 dional canals send off branches which unite ~/~ 

 with one another, forming a complex network | j 

 of tubes, and at their oral ends the four meri- 

 dional canals of each (right and left) side and 

 the corresponding stomodseal canal unite into 

 a horizontal tube, which runs parallel with the 

 margin of the mouth. There is no trace of 

 tentacles either in the adult or in the embryonic | E1|X\ 

 condition. K : B9L- hf 



The Ctenophora are usually per- 

 fectly transparent, and quite colour- 

 less, save for delicate tints of red, 

 brown, or yellow, in the tentacles and 

 stomodaeal ridges. Cestus has, how- 

 ever, 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 Bolinct 

 Jiydatina, one of the Lobata, a Cteno- 

 phore which attains a diameter of 



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

 and not more than O5-2 mm. in diameter, it becomes sexually 

 mature, the gonads producing ripe ova and sperms, and the 

 eggs are impregnated and develop in the usual manner. Soon 

 the gonads degenerate, 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. 



tntft 



FIG. 158. Beroe forskalii 



inf. infundibulum ; mth. mouth ; 

 s. pi. swimming-plates. (After 

 Chun.) 



P 2 



