214 The Study of Animal Life part hi 



early associations has been justified in their far-off children, for in 

 this way the many-celled animals began. 



The cell-substance of a Protozoon is living matter, along with 

 nutritive materials which are approaching that climax, and waste 

 materials into which some of the cell substance has disintegrated. 

 The cell has a kernel or nucleus, or more than one, essential to its 

 complete life. There are bubbles of water taken in along with 

 food particles, and in nearly all freshwater forms there are one or 

 two special regions of internal activity, pulsating cavities or con- 

 tractile vacuoles, which become large and small sometimes rhythmic- 

 ally, and may burst open on the surface of the cell. They are be- 

 lieved to help in getting rid of waste, and also in internal circulation. 

 There is a rind in the Infusorians and Gregarines, and shells of flint 

 and lime are characteristic of most Foraminifers and Radiolarians. 



S- Life of Protozoa. — The life -histories of the Protozoa are 

 very varied, but some chapters are common to most. They expend 

 energy in movement ; they regain this by feeding ; their income 

 exceeds their expenditure,' and they grow ; at the limit of growth 

 they reproduce by dividing into two or many daughter - units ; in 

 certain states two individuals combine, either interchanging nuclear 

 elements (in the ciliated Infusorians) or fusing together (as in some 

 Rhizopods) ; in drought or in untoward conditions, or before 

 manifold division, they often draw themselves together and encyst 

 within a sweated-off' sheath. 



The Protozoa often multiply very rapidly. One divides into 

 two, the two become four, and in rapid progression the numbers 

 increase. On Maupas's calculation a single Infusorian may in four 

 days have a progeny of a million. The same observer has shed a 

 new light on another process — that of conjugation, the temporary 

 or permanent union of two Protozoa, which in the ciliated Infusorians 

 involves an interchange of nuclear particles. In November 1885, 

 Maupas isolated an Infusorian (Stylonichid) and observed its genera- 

 tions till March 1 886. By that time there had been two hundred and 

 fifteen generations produced by ordinary division, but since these 

 lowly organisms do not conjugate with near relatives, conjugation 

 had not occurred. The result, corroborated in other cases, was 

 striking. The whole family became exhausted, small, and 

 " senile " ; they ceased to divide or even to feed ; their nuclei 

 underwent a strange degeneration ; they began to die. But 

 individuals removed before the process had gone too far were 

 observed to conjugate with unrelated forms and to live on. The 

 inference was obvious. Conjugation in these Infusorians is of little 

 moment to any two individuals ; during long periods it need never 

 occur, but it is essential to the continued life of the species. " It 

 is a necessary condition of their eternal youth. " 



