Fig. 76.—Types of megascleres. a, Rhabdus (monaxon diactine); b, stylus (monaxon monactine); c, triod (tetraxon triactine); d, calthrop (tetraxon tetractine); e, triaxon hexactine; f, euaster.
BRANCH I. MEGAMASTICTORA
CLASS CALCAREA
Calcarea are marine shallow-water forms attached for the most part directly by the basal part of the body or occasionally by the intervention of a stalk formed of dermal tissue. They are almost all white or pale grey brown in colour. Their spicules are either monaxon or tetraxon or both. The tetraxons are either quadriradiate and then called "calthrops," or triradiate when the fourth actine is absent. The triradiates always lie more or less tangentially in the body-wall; similarly three rays of a calthrop are tangentially placed, the fourth lying across the thickness of the wall. It is convenient to include the triradiate and the three tangentially placed rays of a calthrop under the common term "triradiate system" (Minchin). The three rays of one of these systems may all be equal in length and meet at equal angles: in this case the system is "regular." Or one ray or one angle may differ in size from the other rays or angles respectively, which are equal: in either of these two cases the system is bilaterally symmetrical and is termed "sagittal." A special name "alate" is given to those systems which are sagittal in consequence of the inequality in the angles. Thus all equiangular systems whether sagittal or not are opposed to those which are alate. This is the natural classification.[[214]]
Sub-Class I. Homocoela.
The Homocoela or Ascons possess the simplest known type of canal system, and by this they are defined. The body is a sac, branched in the adult, but simple in the young; its continuous cavity is everywhere lined with choanocytes, its wall is traversed by inhalant pores, and its cavity opens to the exterior at the distal end by an osculum. The simple sac-like young is the well-known Olynthus of Haeckel—the starting-point from which all sponges seem to have set out. Two processes are involved in the passage from the young to the adult, namely, multiplication of oscula and branching of the original Olynthus tube or sac. If the formation of a new osculum is accompanied by fission of the sac, and the branching of the latter is slight, there arises an adult formed of a number of erect, well separated main tubes, each with one osculum and lateral branches. Such is the case in the Leucosoleniidae. In the Clathrinidae, on the other hand, branching of the Olynthus is complicated, giving rise to what is termed reticulate body form, that is, a sponge body consisting of a network of tubules with several oscula, but with no external indication of the limits between the portions drained by each osculum. These outward characters form a safe basis for classification, because they are correlated with other fundamental differences in structure and development.[[215]]
As in Halichondria, and in fact all sponges, the body-wall is formed of two layers; the gastral layer, as we have said, forming a continuous lining to the Ascon tube and its branches. The dermal layer includes a complete outer covering of pinacocytes, which is reflected over the oscular rim to meet the gastral layer at the distal end of the tube; a deeper gelatinous stratum in which lie scleroblasts and their secreted products—calcareous spicules; and finally porocytes.[[216]] These last are cells which traverse the whole thickness of the thin body-wall, and are perforated by a duct or pore. The porocytes are contractile, and so the pores may be opened or closed; they are a type of cell which is known only in Calcarea. It will be noticed that the fusiform or stellate "connective tissue cells" are absent. The layer of pinacocytes as a whole is highly contractile, and is capable of diminishing the size of the sponge to such an extent as quite to obliterate temporarily the gastral cavity.[[217]]
The choanocytes show certain constant differences in structure in the families Clathrinidae and Leucosoleniidae respectively. In the former, the nucleus of the choanocyte is basal; in the latter, it is apical, and the flagellum can be traced down to it (Fig. 77).
