1 68 MANUAL OF ZOOLOGY. 



gating gradually, takes on a worm-like figure. Eventually it 

 grows round the alimentary canal, and, appropriating it, de- 

 taches itself from the Pilidium as a Nemertid provided with 

 the characteristic proboscis, and the other organs of that group 

 of Turbdlariar (Huxley.) 



CHAPTER XXVI. 



NEMATELMIA. 



I. ACANTHOCEPHALA. 2, GORDIACEA. 3. NEMATODA. 



DIVISION II. NEMATELMIA. This section may be considered 

 as comprising those Scolecids in which the body has an elon- 

 gated and cylindrical shape. Strictly speaking, it should 

 include the Nemertida, but the division is not founded upon 

 anatomical characters, and is employed here simply for con- 

 venience. Most of the Nematelmia possess an annulated in- 

 tegument; but there is no true segmentation, and there are 

 rarely any locomotive appendages attached to the body. The 

 majority are unisexual, and parasitic during the whole or a 

 part of their existence. Three orders are comprised in this 

 division viz., the Acanthocephala, the Gordiacea, and the 

 Nematoda. 



ORDER I. ACANTHOCEPHALA. The Acanthocephala are en- 

 tirely parasitic, vermiform in shape, and devoid of any mouth 

 or alimentary canal. They are provided with a kind of snout 

 or proboscis armed with recurved hooks, which is continued 

 backwards into a bandlike structure (ligamentum suspensorium), 

 to which the reproductive organs are attached. " Immediately 

 beneath the integument lies a series of reticulated canals con- 

 taining a clear fluid, and it is difficult to see with what these 

 can correspond if not with some modification of the water- 

 vascular system." (Huxley.) This system of water-vascular 

 canals, however, does not communicate, so far as is known, 

 in any way with the exterior. At the base of the proboscis 

 is placed a single nervous ganglion, which gives off radiating 

 filaments in all directions. 



Besides the presence of a water - vascular system and the 

 absence of any alimentary canal, another point of affinity 

 between the Acanthocephala and the Tceniada has recently 

 been established by the discovery that the adult worm is 



