Vll 



ANNELIDA 



91 



Beproduction. 



jaws, with which the creature, having fixed itself by its 

 mouth-sucker to an animal, makes a small wound; through 

 this it can suck the blood of its victim, which may be almost 

 any creature to which it can attach itself. 



Leeches lay eggs which are surrounded by 

 little transparent capsules, and are deposited on 

 submerged water-weeds, or on the damp 

 ground close to the water. The cocoon 

 is formed in the same way as that of the 

 earthworm, the two ends closing when 

 the leech has slipped backwards out of 

 it. The flattened greenish-brown capsules 

 of the small pond leech, Nephelis vulgaris, 

 are common on the under side of the leaves 

 of water-plants. They are to be found 

 during the summer months, from June to 

 August, and the eggs hatch out in about 

 six weeks. This leech attacks snails. 



In the preceding two 

 otwoms. chapters, only a few of the 

 very many and varied forms 

 of worms have been described, and yet 

 much variety and adaptation has become apparent. The 

 group is one of special interest to the student of evolution also, 

 for it seems possible that in it we may find those forms that 

 will indicate to us some of the first of those steps by which, 

 from such a lowly grade of life, those modifications have 

 arisen which have led to the evolution of tlie back-boned or 

 vertebrate type, now dominant on the earth. 



At present, the problem of the origin of the Vertebrates is 

 quite unsolved, but when we come to consider their char- 

 acteristics, we shall find they show a resemblance to Annelids 

 in the segmentation to be traced in certain of the body 

 tissues ; and the view, that these two groups had a common 

 origin, is tenable in spite of certain serious difficulties, such 

 as the difference in the relative position of the alimentary 

 canal and the main nerve cord of the body ; in the Annelids, 

 as we have seen, the latter runs on the ventral side of the 

 body, whilst in Vertebrates it is dorsal. 



A worm-like, burrowing creature, Balanoglossus, which has 

 some striking points in common with certain of the lowest 



Fig. 47. — Egg Capsules 

 of Nephelis vulgaris. 



a, The marks where the 

 capsule has closed. 



