HELMINTHIASIS 



S89 



An infective mosquito, when biting a man, injects the saHvary 

 sporozoites into his blood and gives him the infection. 



When a mosquito bites a suitable human carrier it receives the 

 macrogametocytes and microgametocytes, which enable the para- 

 sites to undergo sexual development in its body. 



The scheme is: — 



Malaria. 



Parasites. 



Definitive 

 Hosts. 



Definitive 

 Reservoir. 



Infection. 



Inter- 

 mediate 

 Host. 



Inter- 

 mediate 

 Reser- 

 voir. 



Trans- 

 mission. 



Plasmodium 



malaricB, 

 Plasmodium 

 vivaXy 

 Laverania 

 malaricB. 



Aiiopheline 

 mosquitoes. 



Unknown. 



Salivary 

 sporo- 

 zoites. 



Inocula- 

 tive. 



Man. 



Human 

 carriers. 



Macrogame- 

 tocytes and 

 microgame- 

 tocytes 

 from blood. 



Ingestive. 



B. HELMINTHIASIS. 



Trematode Infections. — The trematode infections of man have as 

 their intermediate host a mollusc. The definitive host is a verte- 

 brate, from whom the eggs escape in the urine, the faeces, or the 

 respiratory secretions. These eggs hatch in water, producing a 

 ciliated, actively swimming miracidium, which will enter and 

 develop in some definite genus of mollusc. Chalmers and Pekkola, 

 watching the miracidia of schistosoma, noted that they rapidly 

 entered the known susceptible molluscs, rejecting other genera. 



It would appear that the mollusc is liable to disease and death 

 if too heavily infected. 



It is therefore necessary to note the classification and method 

 of recognition of the known carriers. 



PHYLUM MOLLUSCA Cuvier. 



Synonyms. — PalUata Latreille; Malacozoa de Blainville. 



Definition. — Metazoa with no sign of primitive segmentation, with well- 

 developed distinct coelom (gonad and pericardial), enteron, and hsemoccel, 

 with (or has lost) a radular sac, with peri oesophageal ring; dorsal moiety is 

 the cerebral commissure, and the ventral the labial commissure; and with a 

 dorsal and a ventral nerve trunk. Body wall differentiated into an antero- 

 dorsal, cephalic portion, with the sense organs, a postero-dorsal, the pallium 

 or mantle, which secretes externally a calcified cuticle, the shell, and develops 

 the ctenidia, or respiratory organs, on its lower surface, and a ventral portion, 

 the foot, or organ of locomotion. A veliger, or free trochospere larva, is nearly 

 always present. 



Classification. — The phylum so defined is divided into three grades as 

 follows : — 



