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Show 1886.] POSITION AND CLASSIFICATION O F SPONGES. 565 which the former consist of similar and equal, undifferentiated, cells, which often remain isolated throughout life, whilst the latter pass through a unicellular stage of short duration only, and consist, when adult, of a number of different cells. There is a vast difference between these two groups : the Protozoa are isocellular, whilst the Metazoa are heterocellular. The Sponges are developed in the same way as all other Metazoa and pass through the same well-known embryonic stages-the Morula, Blastula, &c. They consist, when adult, of a great number of differentiated cells. There are flat epithelial cells all over the outer surface and on the canal-wall ; there are collar-cells round the ciliated chambers. There are gland-cells for different purposes, muscular and nervous cells besides ordinary tissue and amoeboid cells in the Mesoglcea or ground-substance, in which also the ova and spermatozoa are developed. It is therefore quite clear that the Sponges are not Protozoa, but Metazoa, and are, in fact, not similar to Protozoa in any way. The Metazoa are naturally divided into two Groups or Grades- the Coelentera, with a simple undivided body-cavity, all the parts of which are in direct connection with one another ; and the Crelo-mata, which have two distinct and entirely separated body-cavities-a gastral cavity and a coelom or perigastric cavity. The Sponges certainly have a simple and continuous body-cavity and no trace of a ccelom, so that they must be regarded as Coelentera. Long before Hertwig established the ccelom theory, Leuckart had already perceived this important fact, and placed the Sponges among the Coelentera accordingly. Although nobody has ever attempted to regard the Sponges as Ccelomata, there has been great opposition, principally among English authors, to Leuckart's opinion. I dismiss the arguments of those who, like James Clark (284-294), Carter (166), and Saville Kent (7/2), regard the Sponges as Protozoa, on the ground that their idea of Protozoa does not harmonize with the generally adopted meaning of the term, for if it did, they could not, as logical thinkers, count the Sponges among them. Their idea of Protozoa comprises the whole Animal Kingdom, because they draw no distinction between isocellular and heterocellular organisms, and of course all Metazoa are, if this distinction be omitted, colonies of unicellular Protozoa. F. E. Schulze (1361) has taken the unnecessary trouble to refute Saville Kent's (772) statements in detail, and to show that the latter had been guilty not only of levity in the philosophical treatment of his work, but also of recording incorrect observations. Some very excellent men, particularly Balfour (17), Biitschli (138), and Sollas (1440), are inclined to consider the Sponges as a separate group equal in value to our groups Metazoa and Protozoa. This arrangement was arrived at without regard to the division of the Metazoa into Coelentera and Ccelomata. They contrast the sub-kingdom Porifera (Parazoa, Sollas) with the subkingdom Metazoa as a whole. With all respect to the most important opinion of Balfour, I still do not see that there is any justification for the establishment of |