NEMATODA. i6i 



Order 4. Malacobdellini. The head is without lateral grooves. 

 The proboscis is without spines. There is a sucking disc at 

 the hind end of the body. The only genus is Malacobdella, 

 which lives (probably) parasitically in sea-mussels. 



Relationships. — Hubrecht maintains with considerable confidence that 

 Nemerteans exhibit affinities with Vertebrates : — 



Thus (i) The firm sheath of the proboscis is compared to the noto- 

 chord ; 

 (2) The ingrowth from in front of the mouth which forms the 

 proboscis is compared to the oral part of the hypophysis or 

 pituitary body ; 



(3) The ciliated slits on the head are compared to gill-slits ; 



(4) There is some plasticity in the arrangement of the two nerve- 



cords, for as they approach ventrally they tend to produce 

 a ventral nerve-cord as in most segmented Invertebrates, 

 while an approximation dorsally would result in a nerve- 

 cord in the position characteristic of Vertebrates. 



THIRD SERIES OF " WORMS"— Nemathelminthes. 

 Class Nematoda. Thread-worms and hair-worms. 



The Nematodes include many parasites, e.g, the thread- 

 worms of children, and the very dangerous Trichina, but 

 many others spend a large part or even the whole of their 

 life in fresh water or in damp earth. They are very hardy 

 animals, able to live for many days with very little food or 

 oxygen, and able to survive prolonged desiccation — in the 

 case of some " paste eels " for fourteen years. Their food 

 consists of the juices of their hosts, or of rotten organic stuff. 



The body is unsegmented, usually cylindrical, and clad 

 in a firm cuticle made by the epidermis. There is never any 

 ciliated epithelium. The longitudinal muscles lie in four 

 distinct groups, two on each side above and below. 



A straight and simple food-canal usually lies in a dis- 

 tinct body-cavity, and consists of a muscular oesophagus 

 sometimes with cuticular teeth, a mid-gut, and a short rectum. 

 A few forms have no gut. 



The nervous system consists of a collar round the gullet, 

 with six nerves extending forward, and other six backward. 

 One runs along the median-dorsal, another along the median- 

 ventral line. Simple eyes are frequently present. 



L 



