

254 BUSH 



embryos. In some instances this appears to be simply an addition 

 over or in front of the first base, and in others an entirely new plate 

 develops, which pushes the primary one forward until it becomes 

 entirely disconnected and ultimately lost. A series showing this 

 interesting feature was found in S. validus V. (pi. XLIV, fig. 14). 

 In some instances this second base appears to be formed by a network 

 of calcareous deposit over the surface of that portion of the operculum, 

 and in others it seems to be composed of minute granules. In some 

 instances the primary plate itself is repeated, as in S. variabilis sp. 

 nov., where the calcareous disk is composed of two layers easily 

 separable into two complete disks (pi. XLIII, fig. 16), and in S. abnor- 

 mis sp. nov., where there were three similar plates, attached one above 

 the other, the operculum itself appearing to be divided into three 

 chambers, the posterior one containing well-developed embryos (pi. 

 XLIII, figs. 24, 28, 29). In S. formosus sp. nov., where nearly the 

 entire operculum becomes a calcareous cylinder, the primary base was 

 seen inside the cylinder (pi. XLIII, fig. 30), when this was severed 

 from its peduncle, and another plate in process of development was 

 found in the expanded end (pi. XLIII, fig. 23), which apparently was 

 to become another operculum ; two complete cylinders have also been 

 found attached one above the other. This and other instances where 

 the brood-pouch, apparently having split along the back and discharged 

 its embryos, was becoming torn away, revealing a calcareous disk be- 

 neath it, points to the fact that in Spirorbis the animal has the power of 

 renewing its operculum on the same side of the body, instead of form- 

 ing a secondary one on the opposite side, as in Serpula, Crucigera, 

 etc. Caullery and Mesnil found a close relationship between the 

 direction of the coiling of the tube and the development of the animal ; 

 that all dextral forms had the operculum on the right side and all 

 sinistral ones on the left side, presumably differentiated from the second 

 branchia. It would therefore seem improbable that any species could 

 turn in both directions, that is, have both a right and left form, an 

 opinion held by some investigators ; hence the direction of the coiling 

 of the tube is of first importance in determining species. 



The embryological development of a number of species has been 

 studied by several authors Pagenstecher 1862 (S. pagenstecheri 

 Qtr. 1865); Agassiz 1866 + Willemoes-Suhm 1871 + Saint- Joseph 

 1894+ Schively 1897 (S. spirorbis L.) ; Claparede 1868 (S. Icevis 

 Qtr.), Fewkes 1885 (S. spirillum L.) ; Saint- Joseph 1894 (S. pusil- 

 loides nom. nov.) and hermaphroditism has been found to be the rule. 

 Nearly all agree that the spermatozoa are carried in the posterior 



