2 LOWER INVERTEBRATES. 



mass of protoplasm capable of taking food and reproducing its kind. Concerning these 

 latter our knowledge is not absolute, and further observation may show that in these 

 a nucleus really exists, a result rendered more probable by the fact that in the Forami- 

 nifera, in which the existence of a nucleus was long denied, that specialization of the 

 protoplasm has recently been discovered. Another feature which frequently occurs in 

 the Protozoa is the contractile vacuole. This is as yet a problematic arrangement, the 

 function of which cannot be said to be decided. There appears in the body a clear 

 vesicle which, sometimes spherical, sometimes irregular and ramified, slowly increases 

 in size, and then suddenly contracts, leaving no trace, and then gradually appears again, 

 only to repeat the operation. It is thought that in some cases these contractile vacu- 

 oles communicate with the exterior, but this has not been proved. In short, there 

 remains a fine field for investigation in the structure and functions of these problem- 

 atical organs, which will be described more in detail in the succeeding pages. 



Food is taken by the Protozoa into the interior of the body, the digestible portions 

 assimilated, and the portions of no use to the organism afterward rejected. In the 

 lower forms all parts of the body seem to be equally adapted for the capture and 

 engulf ment of food, the Protozoan simply crawling around the object; while in the 

 higher there is a distinct portion of the cell set apar*^ for the introsusception of nutri- 

 ment. The character of nourishment also varies, some forms living on vegetable pro- 

 ductions alone, while others absorb any organic bodies, animal or plant, often devouring 

 forms, rotifers, worms or Crustacea, far higher in the scale than themselves. In the 

 higher Protozoa the food is either brought to the part of the body set aside for the 

 reception of food by currents of water created by rapidly moving cilia, while in others 

 the animals which are eaten are in some unexplained manner benumbed by the Proto- 

 zoan and then devoured. When taken into the body the aliment forms a mass slowly 

 circulating through the protoplasm and is known as a food vacuole. 



Reproduction is accomplished in several distinct ways ; by fission, by budding, by 

 encystment, and the subsequent formation of young, in which the act of conjugation 

 frequently plays a part not yet understood. Two and sometimes more individuals 

 unite and form a single mass, and then either separate, or the whole becomes encysted ; 

 but whether this is to be regarded as a true sexual act, or as an obscure something not 

 clearly defined by the term applied to it of " rejuvenescence," has not been settled. 



Four well-marked groups of Protozoa occur ; Monera, Gregarinida, Rhizopoda, and 

 Infusoria. The great German naturalist Haeckel has proposed a third division. Pro- 

 tista, of organized beings to contain forms which cannot be certainly classed with either 

 the animal or the vegetable kingdoms, and here would come the group Monera, together 

 with other clearly closely allied groups which, by common consent rather than by 

 definite character, are usually regarded as belonging to the vegetable kingdom. But 

 though hard and fast lines do not exist in nature, we are compelled to create boun- 

 daries which are frequently as arbitrary as any to be found in geographies, and for the 

 purposes of this series we prefer to consider the Monera as belonging to the animal 

 kingdom, and to ignore the claims of the Protista. 



Class I.— MONERA. 



The Monera, the lowest group of the Protozoa, may be briefly described, following 

 partly the language of HsBckel, as follows : — 



