242 MANUAL OF ZOOLOGY. 



the Echinoderms, A. Agassiz has, however, succeeded in showing that 

 Tornaria is really the young of Balanoglossus, and that it is developed 

 into the adult by a rapid change, not accompanied by any absorption, 

 or casting off, of any portion of the original larva. 



The Nemerteans are mostly marine in their distribution, a 

 few forms inhabiting fresh water, and two (the Tetrastemmce. 

 of the Bermudas and Philippine Islands) being found in moist 

 places on land. They are found from the arctic seas to the 

 equator, most of them being littoral in their habits, though 

 some live at considerable depths. Recently, Mr Moseley has 

 described a peculiar group of Nemerteans under the name of 

 Pclagonemertida, which are pelagic in their habits. These 

 oceanic forms have a broad, gelatinous, flattened body, and a 

 ramified digestive tract, and thus make a near approach to the 

 dendroccelous Planarians. No certain remains of Nemertids 

 are known to occur in the fossil state, though some obscure 

 remains have been referred to this group. 



CHAPTER XXV. 



NEMATELMIA. 



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



DIVISION II. NEMATELMIA. This section may be consid- 

 ered as comprising those Scolecids in which the body has an 

 elongated 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. Vermiform internal parasites, 

 without mouth or alimentary canal, and having an anterior pro- 

 trusible proboscis armed with recurved hooks. Sexes distinct. 



The Acanthocephala are entirely parasitic, vermiform in 

 shape, and devoid of any mouth or alimentary canal. The 

 front end of the body (fig. 119, /) is developed into a retrac- 



