ig6 WORMS. 



suckers. The blood-plasma is colourless. The ova are large 

 and rich in yolk ; the embryos are hatched at an advanced 

 stage, and soon leave the cocoon, which contains no albummous 

 fluid. 



e.g., Clepsinc, Pontobdella, Branchellion. 



2. Gnathobdellidje, in which there is no proboscis, but the pha^nx 

 usually bears three tooth-plates. . The mouth is suctorial. The 

 blood-plasma is red. The ova are small and without much 

 yolk ; the embryos are hatched at an early stage, and swim 

 about in the nutritive albuminous fluid of the cocoon. 



e.g., Hirudo, Heemopis, Hiemadipsa, Aulastoma, Nephehs. 



Appendix (t) 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-oesophageal 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. 



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 repro- 

 ductive 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 



