NEMERTINE WORMS. 683 



fixed (Floscularia), it ends in an adhesive disc or cup, and is not wrinkled nor 

 retractile ; imt in most of the free-living species the foot is retractile within 

 the body and terminates in a couple of nippers which serve to anchor the 

 body in place. In other forms (Pedalion and Hexarthra), the body is pro- 

 vided with pairs of limb-like processes, ending in a fan-shaped tuft of cilia. 

 These two are free-living forms, skipping through the water by means of 

 their limbs and swimming by means of the ciliary wreath. 



The habitat of the " Wheel-animalcules " is tolerably varied. They may be 

 found in ditches, gutters, ponds, and streams ; sometimes attached to stems 

 and leaves of plants ; sometimes creeping upon them ; sometimes roving 

 through the water. Some species, again, are parasitic. A species, for 

 instance, named Drilophaga bucephala, holds by means of its jaws to a water- 

 worm, Lumbriculus, and sucks the animal it clings to. Again, Callidina 

 parasitica is always found adhering to the limbs of the fresh-water shrimp 

 (Gammarus pulex), and merely creeps about the body of its host. And 

 lastly, the genus named Albertia furnishes an instance of internal parasitism, 

 its species, vermiculus, being found living in earth- worms .and slugs. 



The true position of the Wheel-animalcules amongst the Vermes is one 

 of some doubt. But it is important to notice the resemblances that exist 

 between them with their wreath of cilia and the Trochophore larvae of the 

 Polychsetous Annelids. 



NEMERTINE WORMS. 



CLASS NEMERTINEA. 



Although the members of this division are but little known to the ordinary 

 public, many of them, so far as length is concerned, are the most striking 

 of all Vermes. There is, however, a most astonishing difference in size 

 between various species, for while some of those belonging to the genus 

 Linens may attain a length of several yards, the species of other genera are 

 comparatively of quite insignificant dimensions. In other respects, however, 

 there is a close similarity between the various members of the class, the 

 structural characteristics of which may be briefly summarised as follows : 

 The body is not segmented, and its integument is covered externally with 

 cilia. At the front end a head-region may be distinguished by the presence 

 of eyes and of a pair of ciliated pits, which are probably sense organs of some 

 kind. On the lower side of the head opens the mouth, and this leads into 

 a spacious alimentary canal which traverses the body from end to end. But 

 in addition to the mouth, the head is also furnished with a second aperture, 

 from which a retractile proboscis can be shot forth. This proboscis is one of 

 the most characteristic features of the Nemertiries. When retracted it rests 

 in a special sheath lying along the back, above the alimentary canal, and 

 reaching sometimes to the hinder end of the body. In this state the 

 proboscis has been compared to the finger of a glove that has been turned 

 outside in within the part that covers the hand, the latter representing the 

 sheath ; but when proti uded, the organ is comparable to the finger of the 

 glove in its normal situation. This type of proboscis is of not uncommon 

 occurrence in the Vermes, and from its method of protrusion and retraction 

 is often called an "introvert." The nervous system consists of a pair of 

 chords, one running along each side of the body and uniting at its fore end 

 with the brain, which lies above the alimentary canal and beneath the 



