Compensatory Regulation. 29 



Spirorbis, Pileolaria, Filogranula [ ?]) or dorsal to the line of the 

 branchiae (Pomatoceros, Vermilia and others). In the latter case 

 the operculum may further be either at one side of the median 

 line (most species of Pomatoceros and Vermilia) or in the middle 

 line itself (Pomatoceros elaphus, Haswell). 



In Ditrupa there is a large cup-shaped operculum with a naked 

 stalk situated on the left side in line with the branchiae. It occu- 

 pies a position on the median side of the most dorsal branchia 

 of that side (Fig, 8a). I had only four specimens for examination. 

 In all of these the operculum was on the left side, but whether this 

 was merely a coincidence or not it is impossible to say. The tube 

 of Ditrupa subulata lies freely on the sea bottom usually at a con- 

 siderable depth. Its substance is extremely hard and difficult to 

 break. The tube is slightly curved and resembles very much the 

 shell of the mollusk Dentalium. 



The Spirorbis-like forms (Spirorbis, Pileolaria, etc.) have 

 closely coiled tubes attached by the dorsal side to a flat surface. 

 In some the tube is attached for its whole length but in others 

 (some species of Pileolaria) the end may rise up from the level of 

 the surrounding surface. The direction of the coil of the tube is 

 constant for any one species but varies in the different species. 

 Thus dextral and sinstral species are distinguished according as 

 the tube is coiled clockwise or counter clockwise. The dorsal side 

 of the animal is next to the attached surface and the posterior end 

 of the animal upon removal has a pronounced curve to the right 

 or left according as the tube is dextral or sinstral. In the dextral 

 species the operculum is on the right side and in the sinstral on the 

 left so that in all cases the operculum is on the side next to the 

 concave curve of the shell. The number of branchiae varies in 

 the different species from five to twelve, according to Caullery and 

 Mesnil ('96). Of the two species examined by me Spirorbis 

 Pagenstecheri had four branchiae on the left side and three plus 

 the operculum on the right and Pileolaria sp. had five branchiae 

 on the right and four plus the operculum on the left. In both 

 cases the operculum occupies the position of the next to the dorsal 

 branchia on its side, /. ^., the right side in Spirorbis and the left 

 side in Pileolaria. In all members of the group the operculum 

 is in line with the branchiae. It is as a rule much smaller than the 

 opening of the tube so that the animal can retreat to a considerable 

 distance within the tube. There is no si2;n in either of the two 



