Page:Natural History, Mollusca.djvu/119

 The upper or dorsal tentacles are generally short club-shaped organs, with the upper portion frequently bent into an angle. This portion is always set with narrow, oblique plates, usually ten or twelve in number, pointing behind and downward. They are always retractile, but for the most part not sheathed.

The species of this genus deposit their spawn in the spring; the eggs, as already described, being generally arranged in transverse rows, imbedded in a transparent gelatinous ribbon. Doris bilamellata, which I have kept in captivity, always deposited its ribbons on the side of the vessel just beneath the surface of the water. It adheres by one edge, and forms an imperfect spire or cup, the ribbon being bent upon itself, the upper edge or brim leaning a little outward, and being puckered. That of D. tuberculata is very large, the ribbon being three-fourths of an inch high, and wound round in many spiral coils. I have observed it, on the Devonshire shores, depending in a flaccid manner from the under surface of rocks, during the recess of the tide. The embryos in such coils of spawn are immensely numerous. Mr. Darwin thus describes the contents of a spawn-coil of a species of this genus, which he found at the Falkland Isles. "From two to five eggs [yolks], each $3/1000$ths of an inch in diameter, were contained in a spherical little case [chorion]. These were arranged, two deep, in transverse rows,