3 6 



ANIMAL LIFE-HISTORIES 



this segmented region is destined to form the trunk of the worm, 

 while the head-lobe and broad region immediately below it will 

 together form the head. The latter is at first large out of all 

 proportion, but as it grows more slowly than the trunk this 

 ultimately ceases to be the case. For a long time the segments 

 increase in number, the youngest being always the one adjoining 

 the tip of the tail, while the oldest is next to the head. Among 

 other events which take place 'before the free-swimming larva 

 is converted into the adult worm may be mentioned: the sprout- 

 ing of a pair of tentacles or feelers from the head- 

 lobe, loss of the swimming bands of cilia, and degener- 

 ation of the eye-spots. 



There are considerable differences between the 

 trochospheres of different Annelids, especially in re- 

 gard to the number of ciliated bands present, but of 

 these the most constant is the one at the base of 

 the head-lobe described above. In some cases the 

 developing larvae possess bundles of bristles, which 

 differ greatly as to arrangement and length (fig. 882), 

 but in all cases appear to serve the double purpose 

 of balancing the body during swimming, and affording 

 a certain amount of protection from enemies. They 

 Fig. 882. -Larva are secondary larval structures, and not retained by 



of a Bristle-Worm, J 



with bundles of the adult. 



EnuJged l In the Few- Bristled Annelids (Oligockata), which 



include earth-worms and freshwater forms, as also in 

 Leeches (Discophora\ development is direct, there being no larval 

 stage in the life-history. The members of the former group lay 

 their eggs in a case of horny consistency (cocoon) formed by the 

 hardening of a fluid that exudes from a glandular band of skin 

 (clitellum), which is conspicuous in an ordinary earth-worm, where 

 it is placed not far from the front end of the body, and often 

 erroneously regarded as the result of injury by a spade or other 

 implement. 



The cocoons of earth-worms (fig. 883) are deposited either on 

 the surface of the ground, or else buried in the soil. Although 

 a considerable number of eggs are laid in each of them, but few 

 of these it may be only one develop into adult worms, the 

 successful members (or member) of any particular brood using 

 the rest as food. As the eggs are very small, and do not contain 



