i4o. COMPARATIVE ANATOMY 



numerous, but more numerous in the larger and older 

 than in the smaller and younger animals. In the former 

 as many as 150 may be present, and some authors describe 

 an even larger number. The nuclei always lie in the more 

 peripheral part of the endosarc, never in its centre nor in 

 the ectosarc. This constant position of the nuclei is indicative 

 of a differentiation between the protoplasm of the ectosarc 

 and endosarc ; a differentiation which is also expressed in the 

 more granular appearance of the latter. Further, we find 

 that the processes of digestion are accomplished wholly in 

 the endosarc. Actinosphaeria, like all true animals, ingest solid 

 food, and in any specimen one may observe foreign bodies, 

 infusoria, diatoms, algae, etc., lying in the endosarc and 

 surrounded each by a food-vacuole. The formation of food- 

 vacuoles has not been studied with the same care in Actino- 

 sphserium as in Amoeba, but their form and optical characters 

 differ from those of the ordinary permanent vacuoles with 

 which the body is beset, and there can be little doubt that, as 

 in Amoeba, each food-vacuole is secreted round the solid 

 particle contained in it. That the contents of the food- 

 vacuoles exercise a solvent action upon the diatoms, infusoria, 

 and other bodies taken in as food is shown by the various stages 

 of disintegration exhibited by the latter. The darker more 

 granular constitution of the protoplasm of the endosarc is 

 doubtless due in large part to its being loaded with the 

 products of digestion. The endosarc, then, is the seat of 

 the digestive processes, and it may be remarked that the 

 fact that these processes are here carried on in the same 

 region as that in which the nuclei are found, confirms the 

 conclusion supported by many other facts, that the nucleus 

 plays an important part in the function of assimilation in the 

 cell. 



In order that we may understand the manner in which 

 Actinosphaerium captures and ingests its food, which, it must 

 be remarked, consists of active living organisms, we must first 

 consider the structure and action of its pseudopodia. They 

 are not, like those of Amoeba, of constantly changing form, 

 nor are they blunt and lobose, but are semi-permanent 

 structures radiating outwards in every direction, like fine 

 needles. Careful examination with a high power of the 

 microscope shows that every pseudopod is provided with a 



