150 MANUAL OF ZOOLOGY. 



Section B. DENDROCCELA. Intestine branched or arborescent. 

 Body fiat and broad. 



SUB-ORDER II. NEMERTIDA. The Nemertidw, or ' Ribbon- 

 worms,' agree in most essential respects with the Planarida. 

 They are distinguished, however, by their elongated, vermi- 

 form shape, by the presence of a distinct amis, by the absence 

 of an external aperture to the water- vascular system of the 

 adult, and by the fact that the sexes are distinct. 



Reproduction takes place by the formation of true ova, by 

 internal gemmation, or by transverse fission. In Nemertfis, 

 however, the egg gives rise to a larva, from which the adult 

 is developed in a manner closely analogous to that described 

 as characteristic of the Ecliinodermata. The larval form of 

 Nemertes was described by Johannes Miiller, under the name 

 of Pilidium (fig- 41, 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 cili- 

 ated. 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, 

 elongating gradually, takes on a worm-like figure. Eventu- 

 ally 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. 

 1. ACANTHOCEPHALA. 2. GORDIACEA. 3. NEMATODA. 



DIVISION II. NEMATELMIA. This section may be considered as 

 comprising those Scolecids in which the body has an elongated 

 and cylindrical shape. Strictly speaking, it should include 

 the Nemertidce, but the division is not founded upon anatomi- 

 cal characters, and is employed here simply for convenience. 

 Most of the Nematelmia possess an annulated integument ; but 

 there is no true segmentation, nor are there 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 Acantho- 

 cepliala, the Gordiacea, and the Nematoda. 



