PREDATORY ANNELIDS. 



673 



than themselves. For this purpose they are fitted with organs of locomotion 

 in the form of prominent limb-like, bristle-bearing, processes (parapodia); 

 the conspicuous head is supplied with tentacles of various kinds, and 

 generally with eyes, while the gullet, which 

 is protrusible, is armed with horny teeth. 

 Many of these worms are of large size and 

 are composed of a long series of segments, 

 a species, for example, named Eunice 

 (jigantea, is said to have over 400 of them, 

 and to be upwards of four feet in length. 

 But one of the commonest forms upon our 

 coast, known as the Sea-mouse (Aphrodite 

 acuteatct), is a very different looking 

 animal, with a short, broad, more or less 

 oval body, convex above and flat below, 

 like that of a Slug. Its sides are orna- 

 mented with the most beautifully iridescent 

 hairs, and with barbed retractile spines, 

 which serve as a protection against carnivo- 

 rous fish or other enemies. 



An interesting feature connected with 

 some of these Bristle-worms, is that their 

 method of development takes place by 

 means of what is called an alternation of 

 generations. In Autolytus, one of the 

 Syllidse, for example, the worm that is 

 hatched from the egg and grows to maturity 

 is a neuter, and is consequently quite in- 

 capable of reproducing its kind in the 

 ordinary way. But the difficulty is over- 

 come by the formation of a new worm, the hinder end of the old one 

 breaking off and turning either into a male or a female. In some cases 

 more than one individual is produced at a time by this process of division, 

 so that before they sever themselves from the parent stock a regular chain 

 of worms is formed. In another member of this family, namely, the 

 Branched Syllis (Syllis ramosa), which lives in certain sponges, new individuals 

 are produced as lateral branches, as well as one behind the other. Very 

 exceptionally in the Polychseta the young are born alive. In the majority of 

 cases the eggs give rise to a free swimming larva , the Trochophore or Trochosphere, 

 characterised by the presence of one or more belts of long vibratile cilia 

 encircling the body. According to the number and disposition of these belts 

 the larvse have been named ; those that possess several of them being known, 

 for example, as Polytrochous. Sometimes there is but one belt, which may 

 be in the middle of the body {Mesotrochous), or in front of the mouth 

 (Monotrochous), while not infrequently the Monoirochous larva has a second 

 belt surrounding its posterior end, when it is known as Telotrochous. Some- 

 times, again, there is no definite belt (Atrochous), the larva being uniformly 

 covered with short cilia, bearing, in addition, a special tuft of them at the 

 front end of the body. Somewhat similar larvae are found in other groups 

 of Vermes, and are of special interest, as showing possible relationship 

 between them and the Annelida. 



The second order of Bristle- footed worms is named Oligochseta, on account 

 44 



Fig, 3. THE PEARLT NEREIS 

 (Nereis margaritacea). 



a, Entire worm. 



b, Head with jaws, feelers, and eyes. 





