100 M. ARM AND BUFFER. 



pseudopodia of various shapes, which attach themselves to a 

 calyx or to its contained polyp, or more frequently flatten 

 themselves out around the stem so as almost to encircle it. 

 The ectodermic cells of the free extremities fuse into a common 

 protoplasmic mass, which sends out some few pseudopodia. 

 The slow creeping movements of the ends of the Nematocalyces 

 serve to clean the neighbouring polyps — a function accounting 

 for the frequent presence of foreign particles in their ectoderm. 

 The Nematocalyces of Plumularia, moreover, eat up three parts 

 of the animal which has died — the polyp heads, for instance — 

 and are frequently crammed with dead particles which they have 

 ingested. Nematocalyces, therefore, have a double function ; 

 namely, that of scavenging the outside of the animal by 

 creeping along it, and that of eating up useless tissues. The 

 ectodermic cells of the larvae of Setinse also absorb solid mate- 

 rial after enclosing it with their pseudopodia. 



Intracellular digestion by ectodermic cells, however, is not 

 observed frequently, whereas the absorption and digestion of 

 solid particles by mesodermic structures are easily demon- 

 strated. The nutrition of fresh-water sponges, for instance, is 

 carried out by wandering cells, which correspond to the meso- 

 dermic cells of higher animals.^ 



HackeP noticed the fact that particles of indigo when 

 injected into one of the Mollusca (Thetys) are absorbed by 

 the corpuscles of the blood, but, although this contribution to 

 the knowledge of the functions of mesodermic cells is impor- 

 tant, still more may be learnt from the study of processes 

 taking place during the destruction of useless organs in the 

 period of metamorphosis of Invertebrata. 



Taking Auricularia* as an example, we find that resorption- 

 phenomena occur during two stages of its life history, namely, 

 during the assumption of the pupa-stage, when a part of the 

 longitudinal ring of cilia is lost — that is, is disintegrated and 



' Compare, Metschnikoff, • Zeitscbr. f. wiss. Zool.,' vol. xxiv, p. 10, aud 

 T. E. Schulze, ibidem, vol. xxv, p. 258. 

 2 Rudiolarien, 1862, p. 104. 

 •'' Melscb.nikofF, 'Quart. Jouru. of Micr. Sci.,' vul. xxiv, p. 10. 



