222 



LOWER INVERTEBRATES. 



\'iG. 212. - 



■ Tower of castings of PerichtBta 

 near Nice, 



worms. It is a marvellous reflection tbut tlie whole of the superficial mould over any 

 rich expanse has passed, and will again pass, every few years, through the bodies of 

 worms. The plough is one of the most ancient and most valuable of man's inven- 

 tions, but long before he existed the land was 

 in fact regularly ploughed, and still continues 

 to be thus ploughed, by earth-worms. It may 

 be doubted whether there are many, other ani- 

 mals which ha\e played so important a part in 

 the histoi-y of the world as have these lowly 

 organized creatures." 



The amount of the castings is strikingly 

 shown by those earth-worms which belong to 

 the genus Peric/ueta, for these animals deposit 

 their ejections in remarkable towers, which 

 rise like turrets, with their summits often 

 broader than their bases, to a height of two 

 and a half and even three inches. Near Nice, 

 in France, these towers abound in extraordin- 

 ary numbers, and are probably formed by a 

 species naturalized from the east. Mr. Scott 

 complains of the trouble they cause in the 

 botanic garden at Bombay : " Some of the 

 finest of our lawns can be kept iti anything 

 like order only by being almost daily rolled ; 

 if left undisturbed for a few days they become studded with large 

 castings." The period during which worms near Calcutta display 

 such extraordinary activity lasts for only a little over two months, 

 namely, during the cool season after the rains. Ilensen believes 

 that the importance of the earth-worm is not so much in the pre- 

 paration of humus as in making passages for the roots of plants, 

 and he describes the manner in wliich the burrows arc utilized by 

 plants. Indeed we owe to him the demonstration of the relation 

 of the worms to the fertility of the soil, increasing it as just men- 

 tioned. 



During the mating season the earth-worm leaves his burrow, 

 seeking a mate. The eggs are laid in the ground, and are two or 

 three lines in length. Our figure delineates one of them with the 

 enclosed mature embryo, and its top closed by a valve-like struc- 

 ture adapted to facilitate the escape of the worm. The shell 

 generally contains several yolks, but only one of them usually de- 

 velops. It was once erroneously believed that Lwnbricus might 

 be multiplied by mechanical section, but although the front part 

 of a divided worm survives, the back part dies, unless, indeed, 

 when the front includes only the head and a few segments, for 

 then the survival is reversed, the posterior division living on and 

 manufacturing a new head for itself. 



The aquatic Oligochseta (Limicolaj) are very numerous both in species and indi- 

 viduals, and have been separated into four families. 1. Pheeoeyctid^, of which the 



Fig. 213.— Eggs of 

 eartliworm. 



