Genus and Species. 175 



family. All earthworms, for instance, which have a clitellum 

 beginning as far back as at least the twenty-second segment, 

 which possess spermathecse without diverticula, in which the 

 gizzard lies immediately in front of the intestine, and which 

 never have more than eight setae to each segment of the body, 

 are grouped together in the family Lumbricidae. On the other 

 hand in the East, and in Australia, are worms met with which 

 agree in having a large number of — sometimes as many as 

 seventy or more — setae in each segment, the clitellum begins as 

 early as the thirteenth or fourteenth segment, the spermathecae 

 always have one or more accessory pouchlets, and the gizzard 

 is separated from the intestine by a stretch of oesophagus ; 

 these are ranged into a family, Perichaetidae. Families, again, 

 are combined to form orders ; the earthworm belongs to the 

 order Oligochaeta. This order contrasts with the order 

 Polychseta, which includes the marine worms, such as the lug- 

 worm, by possessing no locomotorial processes of the body, 

 containing closely associated bundles of setae, and in a number 

 of characters. Broader divisions still reduce the animal world 

 to a few classes, until ultimately animals can be grouped into 

 two great divisions, the Protozoa and the Metazoa, which will 

 be defined and contrasted later. 



The classification of the animal world adopted in this book 

 will be found to differ from many schemes of classification in 

 vogue. This is because of the uncertainty of our knowledge, 

 and the consequent variability of opinions. The boundaries 

 of many genera, families, etc., are indistinct; a student of 

 zoology soon becomes familiar with what are known as inter- 

 mediate types. The Dipnoan fishes, for example, have retained 

 many fishlike characters, while they have adopted in addition 

 to these amphibian characters. There are, in fact, in nature no 

 sharply marked lines of division ; and, if there appear to be, it 

 is on account of defective knowledge. Until a year ago it was 

 possible to distinguish, by a number of important characters, the 

 leeches from the earthworms ; but, in 1896, the characters of a 

 remarkable leech, Acanthobdella, were more fully made known. 

 This animal, a leech, in many of its peculiarities has setae, like 

 those of earthworms, and its body-cavity is divided into a 



