90 DR. ELIAS METSOHNIKOFr. 



When it became known that all the lower Metazoa, such as 

 Sponges, Coelenterates, and Turbellarians, possessed an intra- 

 cellular digestion, the conclusion was obvious that this mode 

 of nourishment was one of the few characters in the organi- 

 sation of the Metazoa, which had been directly transmitted to 

 them from the Protozoa, and so constituted a link, however 

 small, between the two groups. Now, since the colonial 

 Monads — organisms which most closely resemble the lowest 

 known Metazoa, their embryos and larvae — show no kind of 

 division of labour, no separation into nutritive and locomotive 

 individuals, the question arises whether the lowest Metazoa 

 have not retained the power of using any or all the cells of 

 their body for the purpose of ingesting food. 



In order to answer this question I undertook, in the course 

 of last year, and especially during a six months' residence in 

 Messina, a series of investigations, the chief results of which 

 will be described in what follows. That portion of my work 

 which relates to intracellular digestion in the endoderm itself, 

 I reserve for a future paper. 



I. — Intracellular Ingestion by Ectoderm Cells. 



Sponges, in which, seeing that they are the lowest Metazoa 

 at present known, one would most especially look for some 

 kind of ectodermal ingestion of food, are not suited for detailed 

 observation ; because the ectoderm in the living creature is 

 either invisible or else can be seen only imperfectly. When 

 Krukenberg speaks of a digestion of albuminoid bodies 

 by the " ectodermal covering " of many sponges, it is not 

 evident whether he really means the delicate sheet of flat 

 epithelial cells which constitutes the true epidermis of these 

 animals. In any case, it is not possible, by such experiments 

 as his, to determine the part played by wandering mesoderm 

 cells immediately below the thin ectoderm. Von Lendenfeld, 

 in a recently published memoir on Australian Aplysinidae,^ 



1 " Ueber Coclenterateu der Siidsee," ii, ' Zeitsclir. f. wiss. Zool.,' Bd. 

 xxxviii, 2, 1883, p. 253. 



