We have referred to the endoparasitism of two genera. _Amoebophrya_ lives in several Acanthometrids, and in the aberrant Radiolarian _Sticholonche_ (see p. 86). The attached species are {162}some of them indifferent to their base; others are only found on Algae, or again only epizoic on special Metazoan hosts, or even on special parts of these. Thus _Rhyncheta_ is only found on the couplers of the thoracic limbs of _Cyclops_, and _Choanophrya_ on the ventral surface of its head and the adjoining appendages. [Illustration: FIG. 62.—_Choanophrya infundibulifera._ A, adult; B-D, tentacles in action in various stages; E, tentacle at rest; F, young, just settled down, _a_, _a_, _a_, Tentacles in various stages of activity; _c_, central cavity; _c.v_, contractile vacuole; _em_, ciliated embryo showing contractile vacuole and nucleus; _f_, spiral ridge; _m_, muscular wall of funnel; _n_, nucleus; _tr_, opening of funnel. (A-D, F, modified after Zenker; E, original.)] We owe our knowledge of this group to the classical works of Ehrenberg, Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed much light on _Dendrocometes_, and Hickson has studied its conjugation. Ischikawa[183] has utilised modern histological methods for the cytological study of _Ephelota bütschliana_. René Sand has written a useful, but unequal, and not always trustworthy monograph of the Order,[184] containing an elaborate bibliography. PORIFERA (SPONGES) BY IGERNA B. J. SOLLAS, B.SC. (LOND.) Lecturer on Zoology at Newnham College, Cambridge. {165}CHAPTER VII PORIFERA (SPONGES)[185] INTRODUCTION—HISTORY—DESCRIPTION OF _HALICHONDRIA PANICEA_ AS AN EXAMPLE OF BRITISH MARINE SPONGES AND OF _EPHYDATIA FLUVIATILIS_ FROM FRESH WATER—DEFINITION—POSITION IN THE ANIMAL KINGDOM. Sponges occupy, perhaps, a more isolated position than any other animal phylum. They are not only the lowest group of multicellular animals, but they are destitute of multicellular relatives. They are all aquatic and—with the exclusion of a few genera found in fresh water—marine, inhabiting all depths from between tide marks to the great abysses of the ocean. They depend for their existence on a current of water which is caused to circulate through their bodies by the activity of certain flagellated cells. This current contains their food, it is their means of respiration, and it carries away effete matters. Consequently sponges cannot endure deprivation of oxygenated water except for short periods, and only the hardiest inhabit regions where the supply is intermittent, as between tide marks. This also renders useless attempts to keep specimens in tanks, unless the water is frequently renewed.