We have already alluded to the resemblance between a young Ascon person and a radial tube of Sycon—a comparison which calls to mind the somewhat strange view of certain earlier authors, that the flagellated chambers are really the sponge individuals. If now we suppose each Ascon-like radial tube of Sycon to undergo that same process of growth by which the Sycon itself was derived from the Ascon, we shall then have a sponge with a canal system of the type seen in Leucandra among British forms, but more diagrammatically shown in the foreign genus Leucilla (Fig. 85). The foregoing remarks do not pretend to give an account of the transition from Sycon to Leucilla as it occurred in phylogeny. For some indication of this we must await embryological research.
In Leucandra the fundamental structure is obscured by the irregularity of its canal system. It shows a further and most important difference from Leucilla in the smaller size and rounded form of its chambers. This change of form marks an advance in efficiency; for now the flagella converge to a centre, so that they all act on the same drop of water, while in the tubular chamber their action is more widely distributed and proportionately less intense (see p. [236]).
Fig. 84.—Transverse section through the body-wall of Grantiopsis. d.o, Dermal ostium; fl.ch, flagellated chamber; i.c, long incurrent canal traversing the thick cortex to reach the chamber layer; p, apopyle. (After Dendy.)
Fig. 85.—Transverse section through the body-wall of Leucilla. d.o, Dermal ostium; ex.c, exhalant canal; fl.ch, chamber; i.c, inhalant canal. (After Dendy.)
Above are described three main types of canal system—that of Homocoela, of Sycon, and of Leucandra and Leucilla. These are conveniently termed the first, second, and third types respectively, and may be briefly described as related to one another somewhat in the same way as a scape, umbel, and compound umbel among inflorescences. These types formed the basis of Haeckel's famous classification.[[221]] It has, however, been concluded[[222]] that the skeleton is a safer guide in taxonomy, at any rate for the smaller subdivisions; and in modern classifications genera with canal systems of the third type will be found distributed among various families; while in the Grantiidae, Ute and Leucandra stand side by side. This treatment implies a belief that the third type of canal system has been independently and repeatedly evolved within the Calcarea—an example of a phenomenon, homoplasy, strikingly displayed throughout the group. It is, remarkably enough, the case that all the canal systems found in the remainder of the Porifera are more or less modified forms of one or other of the second two types of canal system above described.
The families Grantiidae, Heteropidae, and Amphoriscidae, all possessing a dermal cortex, are distinguished as follows:—The Grantiidae by the absence of subdermal sagittal triradiate spicules and of conspicuous subgastral quadriradiates; the Heteropidae by the presence of sagittal triradiates; the Amphoriscidae by the presence of conspicuous subgastral quadriradiates.
Two families of Calcarea, possibly allied, remain for special mention—the Pharetronidae, a family rich in genera, and containing almost all the fossil forms of the group, and the Astroscleridae.
The Pharetronidae are with one, or perhaps two exceptions, fossil forms, having in common the arrangement of the spicules of their main skeletal framework in fibres. The family is divided into two sub-families:—