8 THE INVERTEBRATA 



Actually, the structure is then that of a foam, and the meshwork is an 

 optical section of the walls of bubbles or alveoli which contain the 

 liquid constituent. In the nucleus the more viscid constituent is the 

 linin meshwork, the liquid the karyolymph; in the cytoplasm the 

 meshwork is the spongioplasm, and its contents the enchylema?- The 

 gelation to which this structure is due is produced by fixing reagents 

 in many cases in which it does not exist in life. There is, perhaps, no 

 fundamental distinction between the alveoli and the smaller of the 

 spaces known as vacuoles of which so much use is made in the physio- 

 logy of the Protozoa — for storage, as the site of chemical processes 

 such as digestion, for drainage, for hydrostatic functions, etc. The 

 largest vacuoles have often a definite wall of their own. 



The surface of the protoplasm is protected in various ways. 

 {a) Sometimes, as in some amoebae, it is apparently quite fluid. 

 Then, however, there exists upon it an extremely thin membrane, 

 known as the plasmalemma, which has the power of regulating the 

 exchange of materials between the organism and the watery medium 

 in which it lives. Without this power the protoplasm would soon be 

 poisoned or dissolved, (b) In other cases, the surface layer is semi- 

 solidified (gelated) as a visible, firm, but living pellicle. This is often 

 "sculptured " in a pattern, as in Paramecium (Fig. 84 B, C). {c) Inter- 

 mediate conditions connect the pellicle with the cuticle, a close-fitting 

 dead membrane which may be nitrogenous, as in Monocystis, or of 

 carbohydrate, as in many plant-like flagellates. In typical dino- 

 flagellates (Fig. 40 A) it is composed of stout plates of cellulose. 

 (d) Again, there may be a shell from which the protoplasm can issue 

 through an opening. Such a shell may be nitrogenous, as in Arcella 

 (Fig. 59), etc., of a nitrogenous basis with foreign bodies built into 

 it, as in Difflugia and Rhahdammina (Figs. 60, 6 A), of siUceous plates 

 as in Euglypha (Fig. 7), calcareous, as in most foraminifera (Fig. 6 

 B, C), or of cellulose, as in the spores and sclerotium of the 

 Mycetozoa. It is said that mineral shells always contain a groundwork 

 of organic material. They are often composed of several chambers, 

 and may be perforated by numerous small pores. Houses are loose- 

 fitting, wide-mouthed shells (Fig. 38 C). Cysts are temporary shells 

 without opening, {e) Finally, there may be an external lattice, which 

 is pseudochitinous in Clathrulina (Fig. 8) and siliceous in the 

 Silicoflagellata (Fig. 38 F), or a case of calcareous pieces (Coccolitho- 

 phoridae, Fig. 38 E). The siliceous lattice of many radiolarians is part 

 of an internal skeleton. 



The term ectoplasm is applied to any conspicuously differentiated 

 outer layer of the protoplasm, and denotes very different conditions 

 in different organisms — in Amoeba, a stratum which, save at its 



^ The term hyaloplasm has been used in this, but also in other, senses. 



