224 



ZOOLOGY 



small ; tlie other four (.s.pZ. ^j are continued all along the aboral edge of the body. 



The bases of the two principal tentacles (f. ) are large and are enclosed in sheaths, 



and, as in Lobata, numerous small lateral tentacles (l-t) spring from grooves 



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



The young of Cestus (B) resembles a compressed Cydippid svhich undergoes 



gradual elongation in the median plane. 



BeroS, the ]irincipal genus of the Beroida, has the form of a cylinder (Fig. 171), 



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



and occupied entirel}' by the immense mouth (mth.). 

 The greater part of the body is taken up by the 

 huge gullet ; the infundibulum (in/-), pei'-radial 

 », ,B and infundibular canals, &c., all being crowded 



II li® ^"^"^ ^ small space at the aboral pole. The 



•> <*■ meridional canals send oflf branches which unite 



with one anothei', forming a complex network of 

 tubes, and at their oral ends the four meridional 

 canals of each (right and left) side and the corre- 

 sponding stomodffial 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 condition. 



The Ctenophora are usually perfectly 

 transparent, and quite colourless, save for 

 delicate tints of red, brown, or yellow 

 in the tentacles and stomodasal ridges. 

 Cestus has, however, a delicate violet hue, 

 and when irritated shows a beautiful 

 blue or bluish-green fluorescence. Beroij 

 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 



pi. swimming-plates. (After q^^ ^y^^jj regard to BoUna hydatina, one of 

 the Lobata, a Ctenophore which attains a 

 diameter of 25-40 mm. While still in the larval or cydippid con- 

 dition 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 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. 



Fifi. 171.— Beroe forskalii 

 inf. infundibulum ; mill, mouth 



