SIPHON-WORMS 



433 



much commoner native form is the Horse- Leech (Aulastomum 

 gulo], which has smaller jaws and does not attack animals, preying 

 only upon small aquatic animals. Some of the Leeches do not 

 possess jaws, and these are grouped in a special subdivision of the 

 class. Many of them are parasitic upon fish. 



A. 



SIPHON-WORMS (GEPHYREA) 



This group is retained as a matter of convenience to include 

 a number of worm-like animals, little known except to professed 

 naturalists, and some of which appear to be related to Annelida. 

 None of them exhibit other than feeble traces of segmentation. 



The Bristle-Tail (Echiurus) (fig. 267) has a stout cylindrical 

 body covered by tough skin. The mouth is situated on the under 

 surface at the base of a narrow forward 

 projection known as the proboscis, which 

 must be regarded as a head-lobe. It is 

 used both as an organ of locomotion and 

 in procuring food, a deep groove running 

 along its under side to the mouth. Among 

 the characters which show affinity to the 

 Bristle -Worms may be mentioned the 

 possession of setcz, one or two rings of 

 which encircle the hinder end of the body, 

 while there are a pair of hook-like bristles 

 imbedded in the skin not far behind the 

 mouth. Behind them are the openings of 

 two pairs of excretory tubes (brown tubes) 

 comparable to the nephridia of segmented 

 worms. 



The Siphon- Worm (Sipunculus] (fig. 267), numbers of which 

 are sometimes cast up on the shore by storms, burrows in the 

 sand, which it swallows for the sake of the contained organic 

 debris, in this respect reminding one of the earth-worm, to which 

 also our native species have a slight external resemblance, though 

 they differ markedly in the absence of setae and body-segments. 

 Examination of a living Sipunculus shows that the animal pos- 

 sesses the peculiar power of turning the narrow front part of its 

 body outside in (introverting it), this and the reverse process 

 often being rapidly repeated for a considerable number of 



M mouth; s ? s ' s ? tE; EX ' excretoj y 



apertures ; i, intestinal aperture. 



VOL. I. 



28 



