Members of the Phylum Porifera are commonly called "sponges". In terms of body organization they are only slightly more complicated than the unicellular protists, being at what is called the "tissue grade" of organization. This means that their eukaryotic cells are linked together into tissues, but they lack organ systems. Actually, it is not entirely clear that all sponges are actually at the tissue grade. Some species can be completely disaggregated and the individual cells will survive long enough to reassemble themselves into another sponge. Thus they lie at a sort of transition point between the unicellular Protista and the multicellular Animalia, and are sometimes assigned to a Subkingdom of the latter called the "Parazoa".
Sponges do have several distinct types of cells, each with different functions. (See FIG.) The body wall of a sponge is perforated by numerous pores or ostia through which water is drawn into a central cavity called the spongocoel or central cavity. The osculum is a large opening at one end of the sponge through which all the resulting excurrent is evacuated. Ringing the inside of each ostium are choanocytes ("collar cells") which wave flagellae about, generating the incurrent through the pore and capturing food particles as they float by. Food particles are engulfed by the choanocytes and passed along to amoebocytes in the sponge body, which digest the food and wander throughout the sponge supplying nutrient to all other cell types. The sponge body is covered with a skin-like layer of connected cells called epidermal cells, the pores are lined with porocytes. The epidermal cells hold the colony together, but the jelly-like material within the body must be stiffened by spicules, which are calcareous, siliceous, or organic ("spongin") rods with various shapes (see FIG.). These are secreted by another wandering type of cell called a sclerocyte. Reproductive cells are capable of meiosis when sexual reproduction is called for or of mitosis when the colony buds, and are called archaeocytes. If adverse environmental conditions threaten to kill the sponge, these reproductive cells may encyst and wait out the bad times, to divide and reproduce all the other types of cells necessary for a new sponge.
The simplest types of sponges (ascon type) have a single chambered spongocoel and the ostia dump water directly into it. (See FIG.) This only works for small sponges however. As size increases the surface area through which ostia can open increases by the square of the dimensions, but the volume to be fed increases by the cube of those dimensions. The problem is solved by aggregating several ascon modules around a larger spongocoel, which branches into the individual modules. Larger channels feed incurrents to the ostia of each ascon module. This level is called sycon type structure. With further increase in size the surface area/volume dilemma is solved by aggregating sycon type modules about a third-order spongocoel, which branches into the sycon modules and their ascon type components. This third level of complexity is called the leucon (or rhagon) type.
Sponge taxonomy is based upon the types of spicules in the body and their composition. An outline is:
CLASS DEMOSPONGIA -- Monaxons or tetraxons with rays not at right angles. Spongin, silica or both. (Cambrian-Recent)
CLASS HEXACTINELLIDA -- Hexactine or derived forms. Silica. (Cambrian-Recent)
CLASS CALCAREA -- Calcareous spicules or "solid" calcareous walls. (Cambrian-Recent)
CLASS SCLEROSPONGIA -- Basal massive calcareous skeleton similar to those of stromatoporoids (which are assigned to the phylum for this very reason) and to some creatures originally assigned to the tabulate corals. Upper part of living animal with siliceous and spongin spicules. (?Ordovician-Permian [some former "tabulates"]; otherwise Triassic-Recent)
CLASS STROMATOPORATA -- Extinct; but assigned to phylum because of resemblance to sclerosponges. Internal structure more complicated, however (FIG.). (Ordovician-Cretaceous)
Ranges for the individual classes are given above. Sponges are practically useless as guide fossils because their spicular skeletons simply don't usually hold together after death. A few species have been used in very local zonations. Stromatoporoids are a bit more useful because of their solid skeletons, but they are highly restricted to reef and very shallow water environments.
1) Sponges are primarily marine, though there are numerous freshwater species. All are benthic suspension feeders.
2) Calcarea are more common in tropical, shallow, clear waters, or the limestones deposited in such environments, where CaCO3 is easiest to precipitate.
3) Sclerosponges are known from cavities on lower slopes of modern tropical reefs, and most fossil occurrences are consistent with this environment. Stromatoporata are known principally from reef environments as well, and some paleontologists would simply include them in the Sclerospongia.
4) Demosponges include all known freshwater species, but even so are principally marine. They appear in rocks (and modern environments) of all water depths and of all kinds of sediment type.
5) Hexactinellids are currently found primarily in deep waters, and they appear to have preferred such environments since about the late Mesozoic. Prior to that they were quite common in shallow water as well.
TERMS TO BE FAMILIAR WITH:
Amoebocyte -- a sponge cell which wanders throughout the organism retrieving food from the choanocytes, digesting it, and spreading the nutrient about. (FIG.)
Ascon -- simplest sponge structural type. Pore canals dum directly into unbranched spongocoel. (FIG.)
Astrorhizae -- a complex of shallow canals leading up a mammelon to a pore or cluster of pores. (FIG.)
Choanocyte -- Flagellate cell surrounding inner end of the pore canal which generates incurrent by waving the flagella and gathers food particles brought into the spongocoel by that incurrent. (FIG.)
Epidermal Cell -- A cell on the outer surface of the sponge. (FIG.)
Excurrent -- water leaving the sponge at the osculum. (See incurrent.) (FIG.)
Gallery -- The space between two adjacent laminae in a stromatoporoid. Presumably open or filled with tissue in life. (FIG.)
Incurrent -- Water pulled into the sponge at the ostia, for respiration and food collection. Must be evacuated at the osculum. (See excurrent.) (FIG.)
Lamina -- a single horizontal layer in a stromatoporoid. (FIG.)
Latilamina -- A series of laminae in a stromatoporoid skeleton. At the inner edge the laminae are rather far apart, and they become more closely spaced upward. The next latilamina lies abruptly above, and the effect is to suggest growth rings. (FIG.)
Leucon -- Most complicated sponge structure. Spongocoel branches at least twice into smaller subdivisions which are fed by large canals from the sponge surface. (FIG.)
Mammelon -- a raised bump on the surface of a stromatoporoid or a sclerosponge. (FIG.)
Monaxon -- A spicule with one growth axis. ("Monactine means that the spicule grows only in one direction, "diactine" that it grows toward both ends of the monaxon.) (FIG.)
Osculum -- LArge opening of sponge where excurrent evacuates the water brought into the sponge by all the incurrents at the ostia. (FIG.)
Ostium -- Same as Pore. (FIG.)
Pillar -- A vertical supporting rod between two laminae of a stromatoporoid skeleton. May continue across several laminae. (FIG.)
Polyactine -- Spicule type with rays growing in many directions away from a central point. (FIG.)
Pore -- an opening in the outer surface of a sponge, connected by a canal to the spongocoel. Allows water to be pulled into the interior of the organism. A.K.A ostium. (FIG.)
Porocyte -- a cell which lines the pore opening and canal. (FIG.)
Sclerocyte -- A cell responsible for secreting spicules. (FIG.)
Spicule -- a calcareous, siliceous, or organic skeletal element of a sponge. (FIG.)
Spongocoel -- the central cavity of the sponge into which all incurrent water is funnelled. (FIG.)
Sycon -- Structure of intermediate complexity of sponges. Spongocoel branches once into small modules. (FIG.)
Tetractine -- Spicule form in which four rays grow away from a central point, not mutually at right angles. (FIG.)
Triaxon -- Spicule type with three principal growth axes. Triactine spicules have three rays in a plane, hexactine have six, arranged like a toy jack. (FIG.)
Members of the Phylum Archaeocyatha have been variously interpreted as sponges (because of their pores), corals (because of their generally conic shape and septa), as algae (because their radial symmetry is similar to modern dasycladacean green algae), and as Foraminifera (for whatever reason). They have been extinct since early in the Middle Cambrian, so we are really rather ignorant about their relationships and the form of their soft tissues, but the best guess is that they were tissue-grade organisms, intermediate in complexity between sponges and corals.
The skeleton is of calcite, and there is some variation in form on the simple theme of one perforate cone nested inside another, the two being connected by horizontal and/or vertical calcite plates. The cones are referred to as the inner and outer walls, and the connections are called septa (or parieties) if vertical and dissepiments if horizontal (See FIG.). The inner cone encloses the central cavity. Both walls and the septa are perforated with numerous pores. The lower end of the outer cone lacks pores and is called the aporous tip. It serves as an attachment point for the organism, bearing root-like structures called holdfasts. The space between the inner and outer walls is the intervallum, and individual segments of it between adjacent septa are loculi.
Archaeocyathids are divided into two classes, the Regulares and the Irregulares, based on the relationships of the inner and outer walls to the aporous tip, and on the arrangement of pores, particularly on the septa.
Archaeocyatha are known only from Lower and Middle Cambrian rocks. They appeared earlier than trilobites, being, in fact, among the first organisms with a CaCO3 shell. Russian paleontologists have produced an excellent biostratigraphic zonation of the Cambrian rocks on the Siberian Platform using archaeocyathids.
Archaeocyathids were gregarious creatures, growing close to each other and mutually "climbing up" each other and thus producing mounds on the seafloor. This was a radical concept in the Early Cambrian, but became much more common in later times. Whenever organisms intergrow and produce bathymetric highs, we call the structures ecologic reefs. Early Cambrian reefs were produced by archaeocyathids. Period. The animals apparently preferred living where they could build such reefs. They do occur as isolated individuals in limestone and even shaly beds between their reefs, but are smaller and, of course, less abundant there. They seem to have been happiest in 20-30 m of water. All were marine, some apparently tolerating hypersaline water.
TERMS TO BE FAMILIAR WITH:
Aporous Tip -- The bottom end of and archaeocyathid skeleton. Lacks pores and has holdfasts attached. (FIG.)
Central Cavity -- The opening enclosed by the inner wall. (FIG.)
Dissepiment -- A horizontal connecting plate between the walls of an archaeocyathid.
Holdfast -- Rootlike structure used to hold an archaeocyathid in place. (FIG.)
Inner Wall -- The inner cone of an archaeocyathid skeleton. (FIG.)
Intervallum -- The space between the inner and outer walls. (FIG.)
Loculum -- A singel chamber of the intervallum, segregated by a pair of septa. (FIG.)
Outer Wall -- The outer cone of an archaeocyathid skeleton. (FIG.)
Pore -- Opening in the wall or septum. Presumably allowed passage of water. (FIG.)
Septum -- A radial partition connecting the inner and outer walls. (FIG.)
REFER TO THE DIAGRAMS BELOW, AS WELL AS CHAPTER 12 FOR ANATOMICAL INFORMATION ABOUT SPONGES, STROMATOPOROIDS, AND ARCHAEOCYATHIDS