Great doubts still exist as to the real relations and zoological place of the sponges; and though placed here with the Rhizo-poda, there are many considerations which render this collocation objectionable. The opinions of naturalists in this matter are still unsettled, and the whole subject is a Very complex one, so that it will be sufficient to simply allude to one or two of the more important points affecting this question. We have seen that each sponge may be regarded as an aggregate of protoplasmic masses (sarcoids), each of which is morphologically a single "cell." This fact in itself, as pointed out by Haeckel, is an important one, for the typical Rhizopod, such as Amoeba, is to be looked upon as morphologically a single cell leading an independent life, and singly discharging all the functions of vitality. Upon this ground, as well as upon weighty developmental grounds, Haeckel would remove the sponges altogether from the Protozoa, and would place them among the Caelenterata. On the other hand, the individual "sarcoid" of a sponge, if non-flagellate, presents a resemblance to an Amoeba, which is far too striking to be overlooked; whilst the flagellate sarcoids present an equally conspicuous similarity to the flagellate Infusoria. Indeed it is very difficult to see upon what classificatory principle those sponges (such as Halisarca) which have no skeleton can be separated from such compound flagellate Infusoria as Phalansterium, except that the latter is without the canal-system which traverses the protoplasm of the former. There are also many points of affinity between the sponges and the Radio/aria. These considerations appear to be sufficient, in the meanwhile, and in a work of this nature, to justify the retention of the Spongida in the sub-kingdom Protozoa, in which case they find their most natural position in or close to the Rhizopoda. It is to be remembered, however, that they differ from the other Protozoa and agree with the Caelenterata in the fact that the adult is multicellular; that the ovum breaks up on fecundation into a mass of primitive embryonic cells; and that the larva passes through a stage ("gastrula" stage) in which it is composed of an outer and inner cellular layer, enclosing a central cavity, which communicates with the outer world by a single opening. It should be borne in mind, on the other hand, that there are some undoubted Protozoa (e.g., some of the Radiolarians) which have strong claims to be regarded as multicellular organisms.