80 THE CRAG POLYZOA. 



will be found in the apex of the cone ; and from this point it would seem that the 

 growth had started, as from a centre. The foreign particle, however, in all cases that 

 have come under my observation, is of small size, and such as could be readily carried 

 about with the rest of the growth. 



It should also be noticed, that Hagenow ('Die Bryoz. d. Maastr. Kreidebild.,' p. 100) 

 states that he has seen L. semilunaris and L. Goldfussi, attached by the entire surface 

 upon Belemnites, but these, I think, must have been instances of accidental adhesion. 



Besides the peculiar conformation of the polyzoary, one or two other points respecting 

 the general structure of the Selenariida3, should be adverted to. With no exception 

 among living forms, and with very few, if in reality any, among the fossil, these 

 Polyzoa are distinguished by the circumstance that the cells of which the polyzoary is 

 composed are of two kinds, usually differing in size. The existence of these different kinds 

 of cells has, of course, not escaped the notice of every one who has written on the subject of 

 Lunulites ; and although the true nature of the smaller or secondary cells was wholly 

 unknown, they have been variously designated, as, "accessory cells" (" nebenporen "), 

 " cellules avortees," " pores speciaux," &c., whilst some have confounded the opening of 

 the smaller cell with the mouth of the true habitation of the polypide. With regard to the 

 nature of the secondary cells, it has been supposed that they exhibited " a perfect analogy 

 with the chambers in Eschara and Escharina, deemed to be receptacles for maturing 

 gemmules;"^ but as under the latter head two distinct things have been confounded, this 

 analogy, though quite correct in part, morphologically throws no light upon the physiolo- 

 gical function of the organs. 



In fact, the question would probably never have been determined from the examina- 

 tion of fossil forms alone. And as, till very lately, but one living species had been dis- 

 covered, and that described from a specimen in the same imperfect condition that all 

 fossil ones are found in, the solution of the problem was extremely difficult, until an 

 abundant supply of perfect recent specimens, of various species and genera, collected by 

 Mr. M'Gillivray, in the voyage of the "Rattlesnake," allowed it to be decided in the most 

 satisfactory manner. These specimens, which retained the chitinous parts of the skeleton 

 as well as the calcareous, and in which, in fact, even the muscular tissue could be demon- 

 strated after they had been moistened, proved that the smaller chambers are the cells of 

 vibracula, and in the living state contain probably nothing but the muscular apparatus 

 for the movement of the usually very long and strong seta, which has been adverted to 

 above. 



The constant presence, as it would seem, of this organ, and its apparent importance in 

 the economy of the animal, naturally suggest its being employed in the classification of 

 the Family. This idea appears to have occurred to Lamouroux, who, though acquainted 

 only with two or three fossil forms, long ago suggested the subdivision of the Lamarckian 



' Lonsdale, ' Q. Journ. Geol. Soc.,' i, p. 504, 1845. 



