158 ZOOLOGY FOR MEDICAL STUDENTS chap, iv 



Sharks, BranchelUon, also usually a parasite of Elasmobrahch fishes, 

 which has developed branched gills along the side of its body, and Clepsine 

 a common fresh-water leech which instead of surrounding its eggs with 

 a cocoon carries them about attached to the ventral surface of the body 

 and broods over them. 



A point of practical importance to bear in mind about leeches is that 

 being bloodsuckers they are liable to serve as the intermediate host for 

 protozoan blood parasites : they have already been shown to act as such 

 in the case of various trypanosomes of fresh and salt water fish. 



The Polychaeta, Oligochaeta and Hirudinea are the three main 

 subdivisions of the phylum Annelida or segmented worms. The phylum 

 is characterized by a well-marked assemblage of characters. 



Chaetae are usually present — ^local exaggerations of the cuticle. The 

 central nervous system is in the form of a ganglionated ventral cord 

 connected with supra-oesophageal ganglia. A coelomic body-cavity is 

 present, with paired nephridial tubes. The alimentary canal traverses 

 the entire length of the body — mouth and anus being situated normally 

 at its opposite ends. 



The body is metamerically segmented, being divided into a series of 

 homologous somites in which are repeated such- organs as chaetae, 

 coelomic compartment, nephridia, nerve-ganglion. And in correlation 

 with forward determinate movement the front end of the body shows 

 more or less marked differentiation to form a head in which is situated 

 the mouth and in which are concentrated nerve-centres and sensory cells. 



Finally in the case of those annelids which retain the ancestral marine 

 habit there is commonly a free larval stage of the trochosphere type. 



BOOK FOR FURTHER STUDY 



I. General Text-Book 



The Cambridge Natural History, Vol. IL 



