WORMS. 33 



Eyes, auditory organs, and tactile organs are sometimes 

 united to the nervous system, forming primitive sense organs. 



The vascular system is present in some groups, such as the 

 Anndids, Nemertinea, and Gephyrea. The blood is generally 

 yellow or green, sometimes quite clear, and occasionally red. It 

 is red in the earth-worm, in which the vascular or blood system 

 is best studied. It consists of dorsal and ventral blood vessels 

 united by lateral vessels, and it is also provided with paired 

 swollen trunks called "pulsating" or "blood hearts." Worms 

 respire through the whole body surface, except in some marine 

 forms (Ohcetopods), which have specially modified branched 

 giUs found at the base of the limbs. Excretory organs are also 

 present, consisting of paired canals called "nephridia," which 

 may open by a single pore (Nematoda). In the segmented 

 worms, like the earth-*orm, these nephridia are paired in each 

 segment. The nephridia, which are the equivalent of the 

 kidneys in the higher animals, open into the interior or body 

 cavity of the worm by a ciliated funnel, and to the exterior by 

 a small pore on each side of the segment. 



Both sexual and asexual reproduction take place in worms — 

 asexuaUy, by fission and gemmation, amongst the lower forms. 

 In many cases both male and female organs are found in the 

 same individual, when it is known as a hermaphrodite. This we 

 can readily observe in the earth-worm. Many others (most of the 

 Nematoda) have the sexes in separate individuals, the male 

 being, as a rule, much smaller than the female. Both direct and 

 indirect development take place. Many species pass through a 

 metamorphosis, the egg giving rfse to a larval form, which by a 

 series of changes assumes the adult condition. In the parasitic 

 species two hosts are often requisite for development^ and in 

 these parasitic forms the embryonic stage is sometimes capable of 

 asexual reproduction, when the metamorphosis assumes a com- 

 plicated alternation of asexual and sexual generations. The 

 Trematodes or Liver-flukes, and the Cestodes or Tapeworms, have 

 this phenomenon in their life-cycla 



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