Page:Scientific results HMS Challenger vol 18 part 1.djvu/783

Rh (among the Porodiscida), bearing on the disk-margin two, three, or four spongy arms, commonly disposed regularly in the equatorial plane. Here also occurs the peculiar formation of a "patagium," or of an interbrachial spongy framework different from that of the arms, which connects the arms like a web-membrane in the equatorial plane.

The spongy framework exhibits in all these Spongodiscida no remarkable differences, being everywhere composed of fine branched solid siliceous threads, interwoven in all directions, with irregular meshes of very different size.

The central capsule of all Spongodiscida is filled up with the same spongy framework which covers also both its sides. It grows according to the enveloping skeleton, but remains constantly smaller. The form of the central capsule is circular (lenticular or discoidal) in the Spongophacida and Spongotrochida, whilst in the Spongobrachida it enters into the radial spongy arms, developed from the margin of the spongy disk.

Definition.— with a simple circular disk, without radial appendages on the margin (neither solid spines nor chambered arms).