130 



NEMATODA 



[CH. 



5- 



the vagina of the female by inserting two cnticular hairs, developed 

 from the lining of his proctodaeum, called copulatory spicules. 

 It is a most peculiar characteristic of the Nematoda, which they 

 share with the great group Arthropoda, that no 

 cilia are found in any organ of their body. 

 Even the male cells have no flagella but move 

 in an amoeboid manner. This absence of cilia, 

 which are found in every other group of the 

 animal kingdom except Arthropods, and which 

 even occur in many plants, has received hitherto 

 no explanation. It is possibly correlated with 

 the strong tendency of the protoplasm in both 

 phyla to produce cuticle. The eggs of Nema- 

 toda have a structure well adapted for histo- 

 logical investigation, and have been much 

 utilised in researches on the behaviour of the 

 nucleus before and during fertilisation. As a 

 rule the eggs are laid, but there are many 

 species which produce their young fully formed. 

 Comparatively few species are free-living 

 throughout their whole career, but these few 

 are interesting. They are of small size and 

 inhabit damp earth or mud, one family being 

 marine. The mouth is often provided with 

 movable spines and there are frequently eyes. 

 These features suggest a relationship with the 

 Chaetognatha, which, like the Nematoda, have 

 a thick cuticle and only longitudinal muscles ; 

 and the idea receives support from the existence 

 of a group of small marine "worms" called the 

 CHAETOSOMATIDAE, which are probably also to 

 be regarded as free-living Nematodes. These 

 animals have two semicircles of movable bristles, 

 situated one at each side of the mouth, and 

 in addition a double ventral row of similar 

 spines by means of which they creep about. 

 In this feature and in some others these animals exhibit a resem- 

 blance to the Chaetognatha (see pp. 381 384). 



Taking the free-living forms as a starting-point we can arrange 

 the other families of Nematoda in an ascending scale of increasing- 

 parasitism, culminating in a form like Trichina spiralis, which is a 



FIG. 55. Male A scaris 

 lumbricoides, cut 

 open along the dor- 

 sal middle line x 1. 



1. Oesophagus. 2. In- 

 testine. 3. Testis. 



4. Vas deferens. 



5. Lateral excretory 

 canals. 



