16 COMPARATIVE ANATOMY chap. 



develop into a shell by becoming firmer and severing itself from the 

 protoplasm. The structure of the shell of the Dinqflagellata is com- 

 plicated ; in this case alone the shell consists of cellulose (the substance 

 of the membrane of plant cells). Gelatinous envelopes are also widely 

 dispersed ; they are found in Heliozoa, all Radiolaria (Calymna) and 

 many Infusoria, especially in the attached forms. In the last case they 

 are often plastered over with small foreign particles. In the marine 

 Rhizopoda there arise, by secondary impregnation of an originally 

 chitinous membrane with carbonate of lime, and by a further deposit 

 of the same, variously shaped calcareous shells. These have either 

 one chamber {Monotlialaniia) or become many-chambered {Polytha- 

 lamia), with varied arrangement of the chambers. The shells either 

 possess one large aperture for the emission of the protoplasm {Imper- 

 forata), or they are perforated by numerous fine pores {Perforata). 



In Heliozoa pieces of skeleton come into existence by the impreg- 

 nation of an organic substratum with silica ; these generally lie 

 loosely on the sm-face of the soft body. In Claihrulina, however, a 

 globular, much fenestrated, siliceous skeleton is formed. 



The skeletal formations of the Radiolaria, which are rarely 

 absent, show a wonderful diversity of structure. They consist of 

 fenestrated spheres, several of which, connected by means of rods, 

 are often contained one within another, or of regularly arranged 

 radial spicules, or of bivalve shells, etc. etc. "We have to distinguish 

 two altogether different skeletal forms. One consists of silica and 

 never penetrates into the central capsule ; the other consists of 

 an organic substance akin to chitin (acanthin), and is always cen- 

 trogen, i.e. it radiates from the middle of the central capsule 

 (Acantharia). 



In one principal division of the Gh'egarina (Polycistidce) the extended 

 body is divided by a partition into an anterior part (protomerit) and 

 a jjosterior part (deutomerit). Another partition may cut off from 

 the protomerit a front portion serving for adhesion and temporarily 

 provided with hooks, etc. (epimepit). The partitions are, like the 

 cell integument itself, products of the ectoplasm. 



In contrast to the envelopes and skeletons above mentioned we 

 have the cysts or capsules. The formation of capsules and cysts 

 (encystation) takes place in the most varied Protozoa for protection 

 against dessication and putrefaction, after acquisition of food to admit 

 of undisturbed digestion, in hibernation, etc.; very often reproduction 

 (by fission, gemmation, or sporulation) takes place after the formation 

 of, and under the protection of, a cyst. 



IV. Adaptations for Ingestion of Food. 



The organs of locomotion (amoeboid processes, pseudopodia, 

 suctorial tubules, flagella, cilia, etc.) generally also serve for the 

 seizing and sucking in of food, and for the formation of currents which 



