22 INVERTEBRATE MORPHOLOQT. 



uals of which may remain in connection with one another by 

 means of their profusely-branching pseudopodia. Colonies 

 of Actinophrys are also formed in a similar manner, and among 

 the Kadiolaria the forms with rudimentary shells — such as 

 Sphcerozoum, produce, apparently by the division of the cen- 

 tral capsule, numerous individuals which remain in contact. 



A modification of fission known as budding or gemmation 

 also occurs in some forms. It differs from fission only in 

 that the products of the division differ in size, so that it is 

 possible to regard the larger individual as the parent and 

 the one or more smaller ones formed from it by budding as 

 the progeny. The process is, however, fundamentally the 

 same as fission and is a derivative of that process. In Arcdla 

 bud-like processes arise from the periphery of the parent 

 protoplasm, separate, and assume amoeboid movement leav- 

 ing the shell in an Amoeba-like condition, and it seems prob- 

 able that the marine Foraminifera and certain Heliozoa re- 

 produce in a similar manner. 



Spore-forma,tion also occurs, the parent protoplasm break- 

 ing up more or less completely into a number of small por- 

 tions termed spores, which later increase in size and assume 

 the characters of the parent. This process is sometimes pre- 

 ceded by encystment, a phenomenon not, however, in its origin 

 connected with reproduction. It is more prevalent among 

 fresh-water than among marine forms, and seems to have 

 been originally developed as a protection from injurious ex- 

 ternal conditions, such as the drying up of the pools in which 

 the organisms live. When about to encyst an Ammha, for 

 instance, withdraws its pseudopodia and assumes a spherical 

 shape, and then secretes a more or less dense chitinous case 

 or cyst which completely encloses it. In virtue of the resist- 

 ent and non-conductive nature of the cyst the organism may 

 while in this state, suffer uninjured prolonged exposure to 

 conditions which would quickly entail the death of the non- 

 encysted individual, and on the return of favorable condi- 

 tions may leave the cyst and reassume its active life. Occa- 

 sionally, too, encystment may occur as the result of good 

 nutrition, an individual which has engulfed a number of 

 diatoms, for instance, secreting a cyst around itself witliin 



