ANNULOIDA I SCOLECIDA. 193 



developed in a manner closely analogous to that described 

 as characteristic of the Echinodermata. The larval form of 

 Nejnertes was described by Johannes Miiller, under the name 

 of Pilidium (fig. 70, 4). It is " a small helmet-shaped larva, with 

 a long flagellum attached like a plume to the summit of the 

 helmet, the edges and side-lobes of which are richly ciliated. 

 A simple alimentary canal opens upon the under surface of 

 the body between the lobes. In this condition the larva 

 swims about freely; but, after a while, a mass of formative 

 matter appears on one side of the alimentary canal, and, elongat- 

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

 round the alimentary canal, and, appropriating it, detaches 

 itself from the Pilidium as a Nemertid provided with the 

 characteristic proboscis, and the other organs of that group of 

 Turbellaria " (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 Nema- 

 toda. 



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 



N 



