158 Nerve Structure and Force. [April, 



pervious to rains and the fibres of plants, by drawing straws and 

 stalks and leaves and twigs into it, and, most of all, by throwing 

 np such infinite numbers of lumps, called worm-casts, which form 

 a fine manure for grain and grass. "Worms probably provide new 

 soil for hills and slopes, where the rain washes the earth away, and 

 they affect slopes,, probably to avoid being flooded. Lands that are 

 subject to frequent inundations are always poor ; one great reason 

 of this may probably be, because all the worms are drowned."* 



Let us examine the worm more closely. Its colour' is no matter 

 of chance : the dark brownish-black hue, resembling the colour of 

 the soil in which it' lives, gives the best chance of escape from the 

 keen-sighted birds which make it their prey: and of a similar 

 provision we have numberless examples in both invertebrate and 

 vertebrate divisions of the animal kingdom; in the former, for 

 example, the green colour of the grasshopper, the brown hue of 

 the cigale, whose abode is the dried branches of trees. Even the 

 converse may be noted, namely, that colours may be made sub- 

 servient to predatory habits, as we witness in the sandy-coloured 

 Hon, the inhabitant of a dried and parched soil; the striped and 

 brilhant-hued tiger, which lives among the long grass of the jungle ; 

 or the panther, whose spotted hide may be mistaken for part of 

 the leafy foliage among which it lies crouched. 



The firin thick dermal covering and the muscularity of the earth- 

 worm are just such as would enable it to burrow in the moist soil : 

 its thickness is great, and by reducing the size of the abdominal 

 cavity serves to protect the viscera. The nervous system consists of 

 a supra-cesophageal ganglion, united by two fine chords, surround- 

 ing the gullet with the sub-cesophageal ganglion, and thence pro- 

 ceeds a nervous chord along the abdominal aspect of the animal. 

 There are no eyes, for sight is not wanted, but nerve-filaments go 

 to the oral aperture, and to the appendages which proceed from it. 



A circulatory system, consisting of a dorsal and ventral vessel, 

 with transverse side branches, but with no veiy definite current of 

 blood, is best adapted for the habits of an animal subject to sudden 

 and unequal pressure, while the respiratory apparatus is an aqueous 

 sac. lined with vibratile cilia vvithin the abdominal cavity, on either 

 side of* the body. 



The mode of reproduction is by a distinct sexual apparatus, 

 which comes to maturity at fixed times of the year. In every 

 animal there are both male and female organs, although the con- 

 gress of the sexes seems necessaiy to impregnation. 



It should be noticed that in the Annelids we first observe a trace 

 of a "sympathetic nervous system/' of which more will be said 

 when speaking of more highly organized animals. 



The acephalous mollusca are characterized by having a headless 

 * P. 500. 



