i62 WORMS. 



Along each side, between the dorsal and ventral groups of 

 muscles, there is a narrow area, in which runs a thin-walled 

 canal, perhaps excretory. It unites with the corresponding 

 vessel of the other side, and opens anteriorly and ventrally. 

 There is no blood-vascular system. 



The sexes are separate ; the male is the smaller ; it often 

 has copulatory spicules. The reproductive organs are 

 long tubes, opening about the middle of the body in the 

 females, posteriorly in the males. The spermatozoa are 

 not tailed, but slighdy amoeboid. 



The life-history is often complicated, and may exhibit 

 alternation of generations. 



Life Histories. 



1. The embryo grows directly into the adult, and both live in fresh 



or salt water, damp earth, or rotting plants — Enoplidse, e.g., 

 Enoplus, and other members of the family. 



2. The larvae are free in the earth, the sexual adults are parasitic in 



plants, or in Vertebrate animals; e.g., Tylenchus scandens, a com- 

 mon parasite on cereals ; Strongylus and Dochmitts in man. 



3. The sexual adults are free, the larvse are parasitic in insects ; e.g. , 



Mermis. The fertilised females of Sphcerularia bombi pass from 

 the earth into the body-cavity of humble-bee and wasp, whence 

 their larvse bore into the intestine and eventually emerge. 



4. The larvEE are parasitic in one animal, the sexual adults in another 



which feeds on the first. Thus Ollulanus passes from mouse to 

 cat, Cucullanus from Cyclops to perch. 



There are other life-histories, and many degrees of parasitism. The 

 most remarkable form is Angiostomum (or Ascaris, or Leptoderd) 

 nigrovenosum. In damp earth males and females occur, the progeny of 

 which pass into the lungs of frogs and toads. There they mature into 

 hermaphrodite animals (the only instance among Nematodes), which 

 produce first spermatozoa and then ova. They are self-impregnating, 

 and the young pass out into the earth as males or females. In this case 

 there is alternation of generations, and a somewhat similar story might be 

 told of Rhabdonema strongyloides from the intestine of man, and Leptodera 

 appendiculata from the snail. 



There are several quaint reproductive abnormalities ; thus — the female 

 Sphcerularia bombi, which gets into the body-cavity of the humble-bee 

 has a prolapsed uterus, larger than the body ; the male of Trichodes 

 crassicauda passes into the uterus of the female. 



