28 FRESH-WATER RniZOPODS OF NORTH AMERICA. 



bonate of lime, and again he refers them to the hexagonal system. The 

 crystals sometimes occur in notable quantity, even in young specimens of 

 Amoeba, and are quite conspicuous ; at other times, even in what appear 

 to be the same species, they are few and small, or not clearly evident. The 

 crystals, unlike the quartz sand, appear to be an intrinsic element of the 

 animal. 



There is yet little positively known in relation to the modes of repro- 

 duction of Rhizopods, though it is certain that a common mode is by 

 division, as has been observed in some of the naked Lobosa, Heliozoa, 

 and Monera. The opportunities for observing the process have appeared 

 to me to be by no means frequent ; for in all the four years of my study 

 of the Fresh-water Rhizopods, I met with an Amoeba in the state of division 

 in but few instances, and the common Sun-animalcule, Actinophrys sol, but 

 rarely. 



The shell-covered Protoplasts are frequently found in pairs, with the 

 shells mouth to mouth. When the shells are sufficiently translucent to 

 allow the soft parts within to be seen, at times it is observed that these 

 become confluent in one mass, which flows to and fro from one shell to the 

 other. The mass finally dividing again into two, the individuals with their 

 shells become separated without apparent change from the original state. 

 Tin's condition of union of a pair is commonly named 'conjugation', and 

 has been supposed to hold a relation with the act of reproduction, though 

 it has not been positively proved to be so. 



From certain appearances, hereafter to be considered in the special 

 cases, I have suspected that many of the supposed examples of conjuga- 

 tion are really due to reproduction through division. From the appear- 

 ances alluded to, there is reason to suspect that from time to time the shell- 

 covered Protoplasts, under appropriate conditions, — such as are induced by 

 being well fed, so that the animal is in a highly active condition, and the 

 shell is surcharged with sarcode, — quickly project one half the mass of 

 sarcode, which as quickly assumes the shape and size of the parent. The 

 exuded mass at the same time may be supposed to rapidly secrete a shell ; 

 or, if this is composed of foreign matters, as in the case of a Difflugia, it 

 may be immediately formed by the adherence of particles to the exuded 

 sarcode. If such shall be proved to be one of the modes of reproduction 

 of the shell-covered Protoplasts, it is one which we may suppose would 



