16 



INVERTEBRATE MORPHOLOGY. 



FIG. 4. Arcella mitrata 



(after LEIDYJ 



masses of protoplasm containing a nucleus and a contractile 

 vesicle and presenting a slight differentiation into a peripheral 

 more transparent ectoplasm and a central more granular 

 endoplasm in which the nucleus is imbedded. The pseudo- 

 podia are as a rule blunt lobose processes, though in some 



species they are more or less fila- 

 mentous and may even be some- 

 what permanent. The majority of 

 forms, however, secrete a protective 

 shell of varying composition and 

 complexity. In Arcella (Fig. 4) it 

 is chitinous and smooth, and len- 

 ticular in shape, completely sur- 

 rounding the protoplasm, the pseu- 

 dopodia projecting from the cir- 

 cular opening on the flat surface ; 

 in Euglypka it is similar in composition, but sculptured on 

 the convex surface ; in Difflugia the shell is flask-shaped and 

 composed of particles of sand and similar foreign bodies 

 cemented together, while in a large number of forms, es- 

 pecially those which are marine in habitat, the shell is 

 calcareous in composition. 



It is in these forms with calcareous shells that the great- 

 est complexity of structure occurs. In some, such as Gromia, 

 the shell is simple and flask-shaped, the protoplasm pro- 

 truding from the mouth of the shell and covering its entire 

 surface as a delicate layer, from which the long, slender, and 

 frequently anastomosing pseudopodia take their origin. Al- 

 though the pseudopodia are practically permanent in form 

 their protoplasm is continually changing, currents streaming 

 from the body towards the tips of the pseudopodia and re- 

 turning again to the central mass, a constant circulation being 

 thus maintained, and food-particles caught by the delicate 

 pseudopodia conveyed to the central mass, there to be di- 

 gested. A simple shell is, however, comparatively rare 

 among these calcareous forms , more frequently it consists of 

 several chambers, as in Miliola, the chambers varying in size, 

 the first-formed one being the smallest, and, in addition, in 

 very many forms the shell is perforated by minute pores 



