SPONGES 



sponge undergoing an apparent thickening which is in reality due to 

 the formation of folds and their subsequent coalescence, and it will 

 be seen in discussing the canal system that all thickening of tlie wall 

 of the primitive vase-like sponge organism is to be interpreted morpho- 

 logically in a similar manner. Since in Sycons and Heterocoela 

 generally the body usually grows more or less evenly in all parts 

 at the same time that diverticula are being formed all round, the 

 primitive form of a vase is more often perfectly preserved in these 

 sponges than in any others, though subject to variations of form of 

 subsidiary importance, such as the addition of a stalk (Fig. 10) 

 which in the genus Ute reaches a great length. A remarkable 

 departure from the primitive 

 form is seen, however, in 

 G-rantia labyrinthica (Fig. 11). 

 The young sponge of this 

 species has the form of a 



FIG. 11. 



Grantla labyrinthica, Crtr. Three stages 

 of growth. (After Dendy.) 



FIG. 12. 



Leucandra aspera H., natural size. 



stalked cup, with a thick body wall, formed as above described, 

 by folding and coalescence. Further growth of the body wall 

 causes it to be thrown into numerous folds, the edges of which 

 represent the greatly extended oscular margin. Another Hetero- 

 coele sponge of aberrant form, requiring no explanation, is Eilhardia 

 Schulzei (Fig. 13). 



It is evident from the instances that have been adduced, that 

 the changes in the form of the sponge person which result simply 

 from an uneven or local expansion of the surface of the body wall, 

 are numerous and often complicated, but may, however, result in a 

 simple thickening of the body, and a consequent retention of, and 

 reversion to, the primitive form. 



As a result of a disproportionate increase in the thickness of 



