55. {Chambers long, tubular, branched _Halisarca_ {Chambers not much longer than broad; not branched _Oscarella_ 56. {Amphidiscs present _Ephydatia_ {Amphidiscs absent _Spongilla_ {226} CHAPTER IX PORIFERA (_CONTINUED_): REPRODUCTION, SEXUAL AND ASEXUAL—PHYSIOLOGY—DISTRIBUTION—FLINTS The reproductive processes of Sponges are of such great importance in leading us to a true conception of the nature of a sponge that we propose to treat them here in a special section. Both sexual and asexual methods are common; the multiplication of oscula we do not regard as an act of reproduction (p. 174). [Illustration: FIG. 111.—A, amphiblastula larva of _Sycon raphanus_; B, later stage, showing invagination of the flagellated cells. _c.s_, Segmentation cavity; _ec_, ectoderm; _en_, endoderm. (After F. E. Schulze, from Balfour.)] A cursory glance at a collection of sponge LARVAE from different groups would suggest the conclusion that they are divisible into two wholly distinct types. One of these is the _amphiblastula_, and the other the _parenchymula_. This was the conclusion accepted by zoologists not long ago. We are indebted to Delage, Maas, and Minchin for dispelling it, and showing that {227}these types are but the extreme terms of a continuous series of forms which have all the same essential constitution and undergo the same metamorphosis. The amphiblastula of _Sycon raphanus_ (Fig. 111) consists of an anterior half, formed of slender flagellated cells, and a posterior half, of which the cells are large, non-flagellate, and rounded. These two kinds of cell are arranged around a small internal cavity which is largely filled up with amoebocytes. The flagellated cells are invaginated into the dome of rounded cells during metamorphosis, in fact, become the choanocytes or gastral cells; the rounded cells, on the other hand, become the dermal cells—an astonishing fact to any one acquainted only with Metazoan larvae. A typical parenchymula is that of _Clathrina blanca_ (Fig. 112). When hatched it consists of a wall surrounding a large central cavity and built up of flagellated cells interrupted at the hinder pole by two cells (_p.g.c_)—the mother-cells of archaeocytes. Before the metamorphosis, certain of the flagellated cells leave the wall and sink into the central cavity, and undergoing certain changes establish an inner mass of future dermal cells. By subsequent metamorphosis the remaining flagellated cells become internal, not this time by invagination, but by the included dermal cells breaking through the wall of the larva, and forming themselves into a layer at the outside. [Illustration: FIG. 112.—Median longitudinal section of parenchymula larva of _Clathrina blanca_. _p.g.c_, Posterior granular cells—archaeocyte mother-cells. (After Minchin.)]