2S4 WILSON. [Vol. IX. 



shape with the blade of the shovel. That this is the shape of 

 the shovel blade is gathered from a comparison of the figures 

 a, b, c, Fig. 3', the difference lying in the fact that the trans- 

 verse diameter of the blade // is greater than the dorso-ventral 

 diameter, d. v. The wall of the oval is thickened all round 

 (compare b and c), the thickening being greatest near the ape.x 

 and gradually decreasing towards the equator. A special 

 thickening produces a tooth t on the ventral surface of the 

 blade. The handle of the shovel is directly continuous with 

 the dorsal surface of the blade, and at its ape.x is divided into 

 three small sharp-pointed lobes, one ventral, two lateral. 

 Now a shov'el-shaped spicule of this sort is developed from a 

 small sigma such as d, Fig. 3'. Small sigmas like this and 

 bow-shaped spicules are the only microscleres (smaller spicules) 

 found in the ciliated larva and in the recently attached sponge. 

 The small sigmas are also present in the mesoderm of the 

 adult, and transitional forms between them and the shovels are 

 found. The sigma appears to develop into the shovel in this 

 way. It increases in length and one of the ventral arms, v' in d, 

 becomes relatively long while the other 7'" grows shorter. The 

 longer arm and the axis back of it then flatten out, and grow 

 in such a way as to become connected at the sides forming the 

 blade of the shovel. The other half of the axis does not flatten, 

 but remains as the handle, the shorter arm of the original 

 sigma persisting as the ventral lobe of the handle apex, the 

 other two lobes being formed as outgrowths. The larger 

 shovels are no doubt derived from the smaller by the produc- 

 tion of lateral notches (/. n.. Fig. 3"), which divide the continuous 

 blade of the small shovel into a dorsal (d. /., Fig. 3") and a 

 ventral lobe {v. I., Fig. 3"), the tooth / remaining as a thicken- 

 ing of the ventral lobe. The only other observations I am 

 aware of, on the development of this class of spicules, are 

 contained in Ridley & Dendy's Challenger Report on the 

 Monoxonida (21, p. xx). Their account anticipates mine in 

 the chief point, viz. that the chelae are produced by the gradual 

 alteration of sigmas. 



The study of the anatomy of such a sponge as Esperella 

 would of itself lead one to homologize pores with oscula, and 



