806 ANIMALS AND THEIR ENVIRONMENT 



pod, usually a mosquito. The eggs are ready to hatch in eight or ten 

 days. When the mosquito feeds, the warmth of the mammal stimulates 

 the maggots to emerge and drop onto the host. 



Some of the parasitic copepods (class Crustacea) are attached to the 

 host by their antennae while they feed upon the host with the mouth- 

 parts. In other species the antennae grow into the host to serve as an 

 anchor, and in still others this anchor serves as a nutritive organ and 

 soaks up nourishment from the host. Finally, in several groups of para- 

 sitic copepods the mouth parts are degenerate and the antennae form 

 a root system that spreads throughout the host. In the barnacles this 

 type of parasitism has developed directly from nonparasitic forms. 

 Barnacles usually attach to inanimate objects, but a few species attach 

 to other organisms. In some of these the attachment organ, the antenna 

 of the larva, extends into the host as an anchor, and in other species 

 it becomes a nutritive organ. In some species of both groups the root 

 system becomes much developed while the body left outside degen- 

 erates completely, giving rise to endoparasitism. 



363. Parasites of the Digestive Tract 



These can be divided into several categories: those that eat the 

 host's food, those that are saprozoic, soaking up food the host has 

 digested, those that feed on the digestive tissues and those that suck 

 blood. Intestinal organisms feeding on bacteria are usually commensals 

 and do little or no harm to the host. 



Intestinal parasites that compete with the host for food may cause 

 malnourishment. Nematodes are the most numerous of these parasites. 

 As far as we know, all nematodes swallow food, and many species live 

 in the small intestine eating partially digested material supplied by the 

 host. They are often harmless in the sense that the host can usually eat 

 enough for everybody, but if they become too numerous or if the host is 

 starved the host suffers. Ascaris lumbricoides is so prevalent throughout 

 the world that Chandler has described it as "one of man's most faithful 

 and constant companions from time immemorial." Most mammals have 

 their species of ascaris-like roundworms and it is unusual to open a 

 mammalian intestine and not find them. 



Saprozoic intestinal parasites live in the small intestine where 

 food is digested by the host. The tapeworms (class Cestoda) and spiny- 

 headed worms (phylum Acanthocephala) are the two large groups of 

 such parasites. A number of flagellates are also saprozoic. In man the 

 flagellate Giardia lavibUn (Fig. 39.8) applies its concave ventral surface 

 to an intestinal cell and attaches by suction. It feeds by absorbing 

 nutrients from fluid that is swept past by the flagella. If this species 

 is so abundant as to carpet the gut wall, absorption by the host may 

 be impaired. Tapeworms attach by suckers or hooks and spiny-headed 

 worms bury the head in the intestinal wall. Both groups lack digestive 

 tracts and soak up nutrients through the integument. Their major harm 

 is in the injuries caused by attachment, which may become infected and 



