141 



OTHER ROUND WORMS 



Though many nematode worms are known, none of them 

 has been found to differ anatomically from Ascaris in any import- 

 ant respect : this is remarkable, because while some are parasitic 

 in vertebrates, others live in invertebrates, some parasitise plants 

 and yet others are free-living, so that one type of structure serves 

 for a wide variety of habitats. Their life-histories, however, are 

 as diverse as they are remarkable, probably because it is only 

 by strange and various devices that they can obtain entry to their 

 several hosts. The following are brief outlines of examples of the 

 principal t^^pes of nematode life-history : — 



1. Free-living throughout life. — One of the best-known examples 

 is Rhahditis aherrans (Fig. 104), which is about one mm. long and 

 is common in soil. The adult is easily observed as a transparent 

 object under the microscope, when it can be seen that the oeso- 

 phagus shows active muscular pumping movements while the 

 contents of the mid-gut are moved only by the general contractions 

 and bendings of the body in locomotion. The general anatomy does 

 not differ much from that of Ascaris, but the genital organs are 

 simpler. Copulation having taken place, fertihsation occurs in 

 the upper part of the uterus, and the eggs are laid. They develop 

 into larvse which undergo the four moults usual in the group. 

 After the second of these the larva may remain within the shed 

 skin, in an apparently encysted condition. It retains the power 

 of movement, but can resist desiccation and so serves as a dis- 

 tributive and dormant stage. Larvae in this state are attracted 

 by any strong concentration of decaying organic matter such as 

 a piece of meat placed in the soil, and on feeding on this they 

 rapidly develop into sexually mature adults. In some species of 

 Rhahditis the males are few and sexually degenerate, while the 

 females have developed into protandrous hermaphrodites. 



Another free-hving species is Anguillula aceti, found in vinegar. 



2. Free as larvce, parasitic on plants as adults. — The Cockle 

 Worm, Anguillulina {^Tylenchits) tritici (Fig. 105) causes 

 ear-cockles in wheat. A pair, living in a single flower of the 

 plant, become mature in late summer, and produce hundreds 

 of larvae. The plant tissues react by forming a gall (the 

 cockle) instead of a proper fruit, and in this the larvae may 

 survive for at least twenty years. In damp earth, however, they 

 become active and escape. In the spring the larvae enter the 



