CHAPTER VII 



CHAETOGNATHA 



STRUCTURE -REPRODUCTION HABITS FOOD- 

 TABLE OF IDENTIFICATION 



CLASSIFICATION 



At certain seasons and at certain times of the day the naturalist 

 who is investigating the fauna of the surface of the sea is apt to 

 find his tow-net crammed with innumerable trans- 

 parent spindle-shaped animals, which by their 

 number and the way in which they become 

 entangled with rarer objects, often render useless 

 the result of his labours. These animals belong- 

 to the class Chaetognatha, which includes three 

 genera, Sagitta, Spadella, and Krohnia. Amongst 

 them are divided about twenty species, some of 

 which, however, are of doubtful value. 



Anatomy. The body of these animals is as 

 transparent as crystal ; it is elongated, and bears 

 a resemblance to certain torpedos, except that the 

 head forms a somewhat blunt termination to the 

 spindle-shaped body. The tail bears a caudal 

 fin, and Spadella and Krohnia have a single pair, 

 and Sagitta two pairs, of lateral fins ; all of which 

 are flattened horizontally. 



The body is externally divisible into three 

 regions head, trunk, and tail and these corre- 

 spond with the arrangement of the internal 

 organs, 

 is surrounded by a fold of skin, forming a hood, 



Fig. 101. Sagitta 

 bipunctata. a, 

 Vesicula semi- 

 nalis. x 4. 

 (After Hert- 

 wig.) 



The head 



