PHYLUM COELHELMINTHES (ANNELIDA) 147 



down into the archenteron dividing this cavity into three parts. 

 Of these, the two lateral cavities, lying between each fold and 

 the body wall, later become the coelomic cavities, while the 

 central space between the two folds constitutes the mesenteron. 

 During the gastrula stage, two entoderm cells opposite the blasto- 

 pore become recognizable as the rudiments of the gonads. By 

 a single division, these two cells form two pairs of which the 

 anterior pair later forms the female gonads and the posterior 

 pair the male gonads. The young resemble the parents except 

 in size. 



Sagitta, the arrow-worm, is the most characteristic genus of 

 the Chaetognathi of which Spadella and Krohnia are other 

 recognized genera. 



Class Chaetopoda 



The Chaetopoda are among the most characteristic of Coel- 

 helminthes. Metamerism is sharply marked. The coelom is 

 divided into successive chambers by transverse septa which 

 correspond with the external constrictions of the body wall, and 

 even internal organs such as nervous, excretory, and circulatory 

 systems bear the marks of metamerism. Most of the segments 

 bear bristles or setae which, by their number and arrangement, 

 give a basis for classification into subclasses. In the Polychaeta, 

 the setae occur in outgrowths of the body wall called parapodia 

 which function as oarlike organs in swimming. 



The digestive system, though a straight tube, usually shows 

 specialization into regions. The mouth typically lies on the 

 ventral surface very near the anterior extremity beneath the 

 terminal somite called the prostomium. In the two subclasses, 

 the prostomium is highly variable in degree of specialization. 

 Among the Oligochaeta it is frequently a small, inconspicuous 

 lobe with generalized sense organs similar to those found on 

 other segments of the body, while among the Polychaeta it more 

 frequently bears highly specialized tactile organs and eyes. 



The circulatory system consists of at least two main longi- 

 tudinal trunks, one dorsal and the other ventral, connected by 

 lateral vessels in each segment. Frequently, additional longi- 

 tudinal vessels occur. Some of the lateral vessels in the anterior 

 region of the body are specialized as pumping organs or hearts. 

 These, with the dorsal vessel, propel the blood through the system 

 by their pulsations. 



