SEGMENTED WORMS 



361 



much food-yolk (see p. 345), some such arrangement is necessary. 

 It is true that a nutritive albuminous fluid is enclosed in the 

 cocoon, but this is unfortunately not abundant enough to prevent 

 the more vigorous embryos from becoming cannibals. 



The small cocoons of freshwater worms are commonly attached 

 to aquatic plants, in many cases near the edge of the native pond 

 or stream. It sometimes happens that a considerable number of 

 young worms emerge from each capsule, but this is exceptional. 



The eggs of Leeches {Discophora) are usually enclosed in 

 cocoons, which are generally deposited in damp places near the 

 water, but not actually in it. Some 

 species, however, do not form co- 

 coons at all, but lay their eggs in 

 gelatinous clumps attached to 

 various objects under water, and 

 the cocoons of other leeches are to 

 be found in similar situations, e.g. 

 those of the marine form Potob- 

 della. We should scarcely expect 

 to find "nursing" of young among 

 Leeches, but this actually does take 

 place in certain instances (species 

 of Glossiphonia). Speaking of this 

 sort of leech Beddard says (in The 

 Cambridge Natural History) that it " carries its eggs upon the 

 ventral face of the body, where the young remain for some time 

 after they are hatched, attached by the posterior sucker to their 

 parent's body, and from which situation of safety they make short 

 excursions ". 



The life-histories of animals belonging to the huge group of 

 Jointed- Limbed Invertebrates (Arthropoda) include so many 

 interesting and varied types that it may be well to consider 

 separately the different subdivisions, i.e. Crustaceans (Crustacea), 

 King-Crabs (Xiphosura), Primitive Tracheates (Prototracheata), 

 Centipedes and Millipedes (Myriapoda), Spider-like Arthropods 

 (Arachnida), and Insects (Insecta). 



Up to this point the present section has dealt with the life- 

 histories of Lower Invertebrates, in which the comparatively 

 simple adult stage is reached after a correspondingly simple de- 

 velopment. And though, as we have seen, there are many 



Fig. 883. — Cocoons of Earth-Worms, naturalsize 

 and enlarged, a, Lumbricus rubellus. B, Aliurus. 

 c, Allolobophora fcetida. 



Vol. III. 



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