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 Nectonema 

 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 seems best 

 to include them with the 

 other invertebrates of more 

 or less uncertain systematic 

 position. 



Their resemblance to the 

 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. 



Fig. 119. — Gordius (of the group 

 Nematomorpha) twining around a 

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

 and string of eggs. (From the Cam- 

 bridge Natural History, after von 

 Linstow.) 



