222 



SEGMENTED WORMS OR ANNELIDA. 



Appendix (i) to Annelid Series. 



Class CH^TOGNATHA. Arrow Worms. 



There are two little marine " worms," Sagitta and Spadella, which 

 are so different from all others, that they have been placed in a class by 

 themselves. It is possible to regard them as Annelids with three 

 segments. 



The translucent body, which is about an inch long, has three distinct 

 regions, a head bearing a ventral mouth with spines and bristles 

 (whence the name Chsetognatha), a median region with lateral fins, and 

 a trowel-like tail. 



The nervous system consists of a supra-cesophageal ganglion in the 

 head, a sub-cesophageal about the middle of the body, long commissures 

 between them, and numerous nerves from both. There are two eyes and 

 various patches of sensitive cells. 



The food canal is complete and simple ; it lies in a spacious ciliated 

 body cavity, which arises in the embryo as two pockets (coelome pouches) 

 from the primitive gut cavity or archenteron. Corresponding to the 

 external divisions, the cavities of head, body and tail are distinct. 



P^IG. 72. Development of Sagilta (after O. HERTWIG), 

 illustrating formation of a body cavity by pockets from the 

 archenteron, and early separation of reproductive cells (R.}. 



EC., Ectoderm ; En., endoderm ; ac., archenteron ; ft., reproduc- 

 tive cells; bl., blastopore ; cp., coelome pouches; ;;/., mouth; 

 i. section of gastrula ; 2 and 3. origin of coelome pouches. 



There is no vascular system, nor are there any certain nephridia. It 

 is possible that the latter may be represented by the genital ducts. 



The animals are hermaphrodite, and the simple reproductive organs 

 lie near one another posteriorly. The two ovaries project into the body 

 cavity, and their ducts open laterally where body and tail meet. The 

 two testes project into the cavity of the tail ; and their ducts have internal 

 ciliated funnels, and open on the tail. It is interesting to know that 

 two reproductive cells are set apart at a very early stage, and that each 

 divides into the rudiment of an ovary and of a testis. 



The development is very regular. The eggs undergo complete seg- 

 mentation ; a gastrula is formed by the invagination of a hollow ball of 

 cells ; the body cavity arises in the form of two pockets from the gastrula 

 cavity or archenteron. 



