ZOOLOGY AND BOTANY, MIGKOSCOPY, ETC. 863 



Milne-Edwards has remarked that below 2000 metres the fauna 

 changes. Observations should now be made on the characters of tbe 

 animals that are brought up dead from great depths ; the causes ought 

 to be comparable, though of course of a converse nature, to those that 

 operate on animals subjected to artificial compression. 



B. INVERTEBRATA. 



Intracellular Digestion of Invertebrates.* — E. Metschnikoff 

 insists on the necessity of collecting physiological as well as morpho- 

 logical evidence before discussing the evolution of any system of 

 organs. The author offers some answers to the question 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, and commences 

 with ingestion by ectodermal cells. 



Sponges, unfortunately, are not well suited for observations on the 

 activities of ectodermal cells, and as yet there is no very definite 

 evidence in favour of intracellular digestion by the ectodermal cells of 

 those Metazoa. If powdered carmine be suspended in the water sur- 

 rounding a Plumularia, a considerable quantity will soon enter the 

 substance of the ectoderm by the nectocalyces, which send out various 

 kinds of pseudopodia ; in some cases these may be seen eating up, by 

 means of their ectoderm, the dying hydranths of a colony of Plumu- 

 laria ; the food thus taken in remains in the ectoderm and is not 

 passed on into the endoderm. Adinioe also exhibit ectodermal diges- 

 tion, and gastrulfe have sometimes been observed which are asymme- 

 trical in form and dirty in appearance, owing to the ingestion by their 

 outer cells of a large quantity of foreign matter ; the number of 

 particles in the ectoderm diminishes as the gastric pouches become 

 developed. Ectodermal nourishment may also be observed in the 

 ovarian ova of animals whose generative cells are ectodermal, such as 

 Tubularia and Hydra. 



Wandering mesoderm cells perform intracellular ingestion and 

 digestion not only in sponges, but in the larval forms of certain 

 Echinoderms, where they digest the cellular debris of the disappearing 

 organs, and phenomena of this kind are so constant among Echino- 

 derms that they may be regarded as normal and necessary events in 

 the life of their larvse, where they play the same j)art as the osteoclasts 

 of vertebrate bone. The author thinks that the same resorbent function 

 is to be noticed in the larvaa of Ascidians, and may perhaps be found 

 in Arthropods. 



Metschnikoff has extended to Aurelia aurita, Schneider's observa- 

 tion of the resorption of generative products by amoeboid cells in the 

 Hirudinea. In attempting to define the extent of this property of 

 intracellular digestion, the author has studied the transparent and 

 hardy Bipinnaria asterigera, and PhyllirJioe hucephaliim ; giant cells 

 appeared round foreign bodies injected beneath the epidermis, whether 

 they were merely particles of carmine or a drop of human blood. In 



* Arbeit. Zool.-Zoot. Inst. Wiirzburg, v. (1883) pp. 141-69 (2 pis.). 



