ZOOLOGY FOR MEDICAL STUDENTS CHAP, iv 



lU<m, also usually a parasite of Elasmobranch fishes, 



w hich ; 'i-il branched gills along the side of its body, and Clepsine 



:non fresh- water leech which instead of surrounding its eggs with 



a coco. them about attached to the ventral surface of the body 



i. roods over them. 



nit of practical importance to bear in mind about leeches is that 

 Dickers 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. 



I'dlvrhaeta, Oligochaeta and Hirudinea are the three main 

 subdivisions of the phylum ANNELIDA or segmented worms. The phylum 



.racterized by a well-marked assemblage of characters. 

 Chaetae an usually present local exaggerations of the cuticle. The 

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

 ronneru-d with supra-oesophageal ganglia. A coelomic body-cavity is 

 with paired nephridial tubes. The alimentary canal traverses 

 ire length of the body mouth and anus being situated normally 

 at its opposite ends. 



Tin- body is metamurically segmented, being divided into a series of 



homologous somites in which are repeated such organs as chaetae, 



uK-lomic compartment, nephridia, nerve-ganglion. And in correlation 



with forward determinate movement the front end of the body shows 



: 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. GKNKRAI. TKXT-BOOK 

 The Cambridge Natural History, Vol. II. 





