The porifera and coelentera . ply that of a great thickening of the body wall. The Syconsfurnish, in fact, a clear instance of the body wall of the jirimitive SPONGES sponge undergoing an apparent thickening which is in reality due tothe formation of folds and their subsequent coalescence, and it willbe seen in discussing tlie canal system that all thickening of the xoallof the primitive vase-like sponge organism is to be interpreted morpho-logically in a similar manner. Since in Sycons and Heterocoelagenerally the body usuallj^ grows more or less evenly in all partsat the same time that diver
The porifera and coelentera . ply that of a great thickening of the body wall. The Syconsfurnish, in fact, a clear instance of the body wall of the jirimitive SPONGES sponge undergoing an apparent thickening which is in reality due tothe formation of folds and their subsequent coalescence, and it willbe seen in discussing tlie canal system that all thickening of the xoallof the primitive vase-like sponge organism is to be interpreted morpho-logically in a similar manner. Since in Sycons and Heterocoelagenerally the body usuallj^ grows more or less evenly in all partsat the same time that diverticula are being formed all round, theprimitive form of a vase is more often perfectly preserved in thesesponges than in any others, though subject to variations of form ofsubsidiary importance, such as the addition of a stalk (Fig. 10)which in the genus Ufe reaches a great length. A remarkabledeparture from the primitiveform is seen, however, inGrantia lahi/rinthica (Fig. 11).The young sponge of thisspecies has the form of a. Fig. 11. Grantia labyrinthica, Crtr. Three stagesof growth. (After Dendy.) Fig. cispera H., natural size. stalked cup, with a thick body wall, formed as above described,by folding and coalescence. Further growth of the body wallcauses it to be thrown into numerous folds, the edges of Avhichrepresent the greatly extended oscular margin. Another Hetero-coele sponge of aberrant form, requiring no explanation, is EilhardiaSchulzci (Fig. 13). It is evident from the instances that have been adduced, thatthe changes in the form of the sponge person which result simi)lyfrom an uneven or local expansion of the surface of the body wall,are numerous and often complicated, but may, however, result in asimple thickening of the body, and a consequent retention of, andreversion to, the primitive form. As a result of a disproportionate increase in the thickness of lO SPONGES the body wall the primitive vase-like sponge person may assumea more massive form, a
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