First Many Celled Animals : Sponges (1962)

focus units or cell per for this most primitive that accounts for differences in the bulk of individual animal this is as true for this sponge as it is for an elephant or a mouse we can demonstrate the relationship between cellular structure and bulk by using egg white egg white has a consistency much like living protoplasm a large amount of egg white cannot maintain a constant shape but by putting the egg white we form cell like masses of air and albumin roughly these masses may be compared with protoplasm which has been partitioned into cells and enclosed in membranes unlike plain egg white the air and egg white mixture can hold almost any form especially when supported by a skeletal structure such as Excelsior this analogy applies to all many celled animals there is another even more compelling reason why animal bodies larger than protozoans must consist of many cells not just larger masses of protoplasm only small masses of protoplasm can receive sufficient oxygen and dispose of wastes fast enough to support life we can demonstrate how this works here is a cube representing a cell the in going arrows represent oxygen entering the cell oxygen can only enter to expose surfaces of the cell furthermore it can only diffuse into the cell at a given rate of speed the outgoing arrows represent wastes leaving the cell again diffusion is so slow that a large mass of protoplasm simply could not rid itself of waste fast enough to stay alive but if this larger mass has divided into many smaller units with spaces between them much more surface area is a exposed with more exposed surface sufficient diffusion can now take place we may consider this need for many cells in animals larger than protozoans quantitatively by using these model cells this large cube is 3 inches on a side this small cube is 1 inch on a side 27 of the small cubes equal the size of the one large cube by carefully removing a thin paper coating representing the cell membrane from the large cube we can readily observe the total surface where diffusion into or out of this mass could occur in the same way we can remove the surface coating of one of the 27 small cubes finally by comparing the total surface area of all the small cubes with that of the one large cube we can see the enormous increase in surface area relative to volume made possible by splitting the large mass the increased surface area the spaces between the cells and the shorter distance that oxygen and wastes now have to travel make possible a body plan that permits growth beyond the size of protozoans in all animals made up of many cells except the colonial protozoans certain cells are adapted to perform specialized tasks such specialization leads to greater efficiency and the probability of exploiting new sources of energy how does the body plan of a sponge bear out this generalization essentially the sponge is a vase shaped sack with a large opening at the top and microscopic pores perforating the sides food and water these microscopic in current pores because of the small size of these pores the sponge can feed only on microscopic organisms water passes into the main cavity then leaves by way of the large X current opening so there is a continual flow of water through the sponge by using a dye we can see the movement of water through the body of a living sponge the dye moves toward the wall and disappears into the pores of the sponge we can see the dye particles emerging from the large X current opening if these had been food particles the sponge would have retained them the principle types of specialized cells in the sponge are identified in this drawing the sac is covered and protected on the outside by flattened covering cells which fit together like the tiles in a mosaic through the poor cells water is drawn into the sponge the large internal cavity is lined with collar cells the word collar refers to the delicate collar of protoplasm that encircles the free end of each cell this free end also bears a long flagellum whose base passes through the collar it is the beat of the flagella that creates the water current which passes through the sponge as the water passes through the sponge the collar cells capture food organisms between the covering cells and the collar cells are moving mesenchyme cells they receive partly digested food particles from the collar cells complete the digestion and carry the digested food from one part of the sponge to another a function of some of them as income cells is to secrete needles of calcium carbonate these needles are called spicules the spicules form a framework which supports the soft cellular mass much as the Excelsior supported the egg white in our earlier experiment we can detect the presence of calcium carbonate or limestone in the skeleton with dilute hydrochloric acid the acid dissolves out the skeleton of the sponge here are different kinds of spicules have seen under the microscope mesenchyme cells also have a part in the sexual reproduction of sponges some cells enlarge with reserve food and become eggs others enlarge and become sperms sperms are brought inside the sponge in its water current eggs such as the one shown here develop into flagellated larvas the larvas escape from the parent and swim about until they find a place to become attached each surviving larvae then grows into a young sponge sponges may also reproduce asexually by budding and branching sometimes buds are torn off and grow into complete individuals this ability to regenerate living parts is more pronounced in the sponge than in higher forms of animal life this is a freshwater sponge like most freshwater forms its method of reproduction is somewhat different from that of saltwater forms its mesenchyme cells build up an asexual reproduction unit known as a Jim you'll hear is a gem you'll seen under a microscope it consists of a mass of food filled mesenchyme cells surrounded by heavy protective coats strengthened by spicules such gemmules cancer I've drying and freezing under favourable conditions the sponge cells emerge through a thin spot in the gem Yule coat aggregate in a small mass and grow into a new sponge sponges seem to have changed little during their 500 million years of evolutionary history but some variations have developed most evolutionary changes have been in the system of canals and the skeletal structures sponges have few enemies one is the nudibranch some browse on living sponges more dangerous are other living forms that grow over sponges like hydroids or algae such forms of life can choke some sponges by covering up their intake pores silt or mud may also cover the intake pores some types of sponges have adapted to such conditions so that the growth of algae is not harmful in fact it may even protect the sponge from intruders sponges do not respond in any conspicuous way to cutting this apparent immobility does not mean that the sponge is totally incapable of movement there is some movement but it is extremely slow a stimulus does not affect more than a very restricted part of the sponges body occasionally one finds what appears to be a moving sponge a closer look shows it is a sponge growing on a crab the sponge serves to disguise this crab hermit crabs sometimes use sponges as shelters there are more than 5,000 different species of sponges grouped according to the composition of their skeletons some have skeletons of calcium carbonate some have skeletons made of glassy silicate like this Venus's flower basket others consist of silicate combined with sponge and a horny fibrous material still others are made of sponge and alone these are useful as bath sponges such sponges have commercial value and are collected by divers trampling and washing breaks up and kill the living cells leaving the skeleton behind from time to time they are checked to see if they are well cleaned finally the soft skeleton of Spungen appears under a microscope we can see the fine structure of this skeleton which appears as a three-dimensional network of fine threads this network provides strength elasticity and a framework for the cells under the microscope - we can see how water moves through the skeleton of the sponge the water can be removed with the slightest pressure this makes the sponge especially useful in washing biologists have long been interested in the sponges remarkable powers of regeneration we can demonstrate what happens when we take a piece of living sponge and disassociate it cells the cells are squeezed through a bag of fine bolting silk into a dish of seawater a drop of this suspension when examined under a microscope shows that it consists of a large number of separate sponge cells the cells are well disassociated when they settle in the bottom of the dish they begin to wander about in an erratic manner this time-lapse photo micrography shows individual cells joining together into small groups of 6 to 20 plunges as large as a pea are formed this unusual development is significant because the cells have crept together by themselves this mask contains various cell types such as collar cells covering cells and poor cells such experimental work on sponge regeneration not only provides information on the nature of the sponges cellular structure but may also help provide additional knowledge about the mechanisms of wound healing and tissue grafting in other animal bodies the sponge illustrates the development of multicellularity in animals and some of the ways in which the body plan of this primitive animal fits it for survival although the sponge is interesting because it illustrates the cellular level in animal structure a stage no longer found among other many celled animals it is little more than a side issue in the trend of evolution