544 



E. RAY LANK ESTER. 



head is a tagma or group of somites whicli differ in number 

 and in their relative position in regard to the mouth, in 

 different classes. In a simple Chastopod (fig. 1) the head 

 consists of the first somite only ; that somite is perforated by 

 the mouth, and is provided with a prostomium or pra3oral 

 lobe. The prostomium is essentially a part or outgrowth of 

 the first somite, and cannot be regarded as itself a somite. 

 It gives rise to a nerve-ganglion mass, the prostomial ganglion. 

 In the marine Cl)a3topods (the Polychteta) (fig. 2) we find 



Fig. 1. 



Fig. 2. 



Fig. 1. — Diagram of the Lead and adjacent region of an Oligo- 

 clia;te Chsetopod. Pr, tlie prostomium ; m, the mouth ; A, the 

 prostomial ganglion-mass or archicerebrum ; I, II, III, coelom of 

 ihe first, second, and third somites. (From Goodrich, 'Q. J. Micr. 

 Sci.,' vol. xl, p. 247.) 



i'lG. 2. — Diagram of the head and adjacent region of a Poljchsete 

 Chfetopod. Letters as in Fig. 1, with the addition of T, prosto- 

 mial tentacle; Pa, parapodium. (From Goodrich.) 



the same essential structure, but tlie prostomium may give 

 rise to two or more tactile tentacles, and to the vesicular 

 eyes. The somites have well-marked parapodia, and the 

 second and third, as well as the first, may give rise to 

 tentacles which are directed forward, and thus contribute to 

 form " the head." But the mouth remains as an iupushing 

 of the wall of the first somite. 



The Arthropoda are all distinguished from the Cbsetopoda 

 by the fact that the head consists of one or more somites 

 which lie in front of the mouth (now called prosthomeres). 



