INVERTEBRATES OF UNCERTAIN POSITION 179 



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

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

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

 vaginated portion escapes from the PUidium and grows into 

 the adult nemertine. 



3. NEMATOMORPHA 



This group (Gr. ncwa, thread; morplie, 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 FIG. 119. 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 ^idge 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. 



