252 LECTURE XII. 



morphosed germ-cells. And these are not used up. as in higher animals, 

 in developing the tissues and organs of an undivided or individual 

 whole ; but, after a comparatively slight growth and change of the pri- 

 mary segment, proceed, in the typical orders, to form a second segment 

 of somewhat simpler structure, and then repeat such foi-mations, in a 

 linear series, perhaps more than a hundred times. So that we may have 

 a seeming individual anellid consisting of many hundred segments, in 

 which a single segment would give all the characteristic organisation of 

 such individual, except some slight additions or modifications character- 

 ising the first and last of the series. Thus the anellids are more simple 

 in their structure than they appear at first sight, especially before 

 arriving at full maturity, when in certain species, as the leech, nais, 

 and earth-worm, particular segments are distinguished by the deve- 

 lopment therein of the special male and female generative organs. 

 With this characteristic mode of gi'owth by repetition of segments, 

 we should be prepared to expect in these creatures great powers of 

 repair after injuries and mutilations — powers which have been made 

 the subject of many and ingenious experiments. In the works of 

 Spalanzani and Bonnet more particularly are recorded the most 

 striking results of such experiments. A worm cut in two was found 

 to reproduce the tail at the cut extremity of the cephalic half, and to 

 form a head upon the caudal moiety. Bonnet * progressively in- 

 creased the number of sections in healthy individuals of a small 

 worm or nais, which he calls the Lumbricus variegatus ; and when 

 one of these had been so divided into twenty-six parts, almost all of 

 them reproduced the head and tail, and became so many distinct indi- 

 viduals. Sir John G. Dalyell has succeeded in artificially propagating 

 a Sahella in the same way. The small fresh-water naids show great 

 poAvers of repair and reproduction ; these little worms have a paler 

 colour of the blood than in most Annulata ; but through the trans- 

 parent skin you may clearly see its circulation. There are some 

 species found in sand or mud, such as those that stain of a red 

 colour extensive tracts of the Thames mud at low water, which, when 

 submerged, habitually protrude the anterior half of their body, which 

 is remarkable for its regular oscillating movement. Bonnet cut off 

 the head of one of the naids of this genus, which was soon repro- 

 duced ; and, when perfect, he repeated the act ; and again as often 

 as the head was reproduced. After the eighth decapitation the un- 

 happy subject was released by death ; the execution took eftect, the 

 reproductive virtue had been worn out. This series of experiments 

 occupied the Genevese philosopher two summer months. Since many 



* CLXXXn. pp. 117.245. 



