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 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.
Fig. 112.—Median longitudinal section of parenchymula larva of Clathrina blanca. p.g.c, Posterior granular cells—archaeocyte mother-cells. (After Minchin.)
In the larva of C. blanca, after a period of free-swimming existence, the same three elements are thus recognisable as in that of Sycon at the time of hatching; in the newly hatched larva of C. blanca, however, one set of elements, the dermal cells, are not distinguishable. The difference, then, between the two newly hatched larvae is due to the earlier cell differentiation of the Sycon larva.[[254]]
Now consider the larva of Leucosolenia. It is hatched as a completely flagellated larva; its archaeocytes are internal (as in Sycon); future dermal cells, recognisable as such, are absent. They arise, as in C. blanca, by transformation of flagellated cells; but (1) this process is confined to the posterior pole, and (2) the internal cavity is small and filled up with archaeocytes. Consequently the cells which have lost their flagella and become converted into dermal cells cannot sink in as in C. blanca: they accumulate at the hinder pole, and thus arises a larva half flagellated, half not; in fact, an amphiblastula. Or, briefly, in Leucosolenia the larva at hatching is a parenchymula, and when ready to fix is an amphiblastula; and, again, the difference between the newly hatched larva and that of Sycon is due to the earlier occurrence of cell differentiation in the latter. What completer transitional series could be desired?
Turning to the Micromastictora, the developmental history already sketched is fairly typical (p. [172]). The differences between Mega- and Micro-mastictoran larvae are referable mainly to the fact that the dermal cells in the latter become at once differentiated among themselves to form the main types of dermal cell of the adult.[[255]] The metamorphosis is comparable to that of C. blanca. Among Tetractinellida and Hexactinellida sexually produced larvae have not been certainly identified.
Asexual reproduction takes place according to one of three types, which may be alluded to as (1) "budding," (2) "gemmulation," (3) formation of "asexual larvae."
By budding (Fig. 113) is meant the formation of reproductive bodies, each of which contains differentiated elements of the various classes found in the parent. A simple example of this is described by Miklucho Maclay in Ascons, where the bud is merely the end of one of the Ascon tubes which becomes pinched off and so set free.