INVERTEBRATES OF UNCERTAIN POSITION 



179 



long' tuft of cilia at the apex. The adult develops from this 

 larva by the formation of ectodermal invaginations (Fig. 118, 

 E, E l ) which surround- the alimentary canal (D). This in- 

 vaginated portion escapes from the Pilidium and grows into 



the adult nemertine. 



> 



3. NEMATOMORPHA 



This group (Gr. nema, thread; morphe, form) contains a single 

 family, the GORDIID.E, and two genera, Gordius, which lives in 

 fresh water, and Nedonema 

 in the sea. They are long, 

 slender thread-like animals 

 (Fig. 119) often found in 

 ditches and commonly called 

 horsehair snakes. Some 

 authors consider them an 

 order of NEMATODA; whereas 

 others rank them as a class 

 under the Phylum NEMAT- 



HELMINTHES. It SCCmS best 



to include them with the 



other invertebrates of more FIG. ug. Gordius (of the group 



or less uncertain Systematic NEMATOMORPHA) twining around a 



. . water-plant and laying eggs, a, a, clump 



position. and string of eggs. (From the Cam- 



Their resemblance to the bridge Natural History, after von 



. ,. Linstow.) 



NEMATODA, indicated by the 



term NEMATOMORPHA, does not hold for the internal anatomy. 

 A distinct epithelium lines the body-cavity ; no lateral lines are 

 present; there is a pharyngeal nerve-ring and a single ventral 

 nerve-cord; and the ovaries, which are segmentally arranged, 

 discharge the eggs into the body-cavity. 



The larvae of Gordius usually migrate into the immature 

 stages of aquatic insects; these are then devoured by other 

 animals in whose intestines the young live and develop until 

 they finally escape into the water. 



