558 INVERTEBEATA CHAP. 



system. The nervous strands which proceed outwards from these 

 centres are at first merely grooves or gutters in the internal walls of 

 the calyx. They become shut off from the coelom and transformed 

 into solid cords of cells which develop sheaths of nerve-fibres. The 

 mesenchyme cells, by which they are surrounded, become calcigenous 

 tissue, and thus the original plates of the young Crinoid are increased 

 in thickness, and in this way the nerves come to lie in the centre of 

 the radial and brachial plates of the adult. Exactly the same 

 process takes place with regard to the cirri; each ampulla of the 

 chambered organ gives off, as we have seen, a branch leading into 

 the cirrus, and this branch develops a nervous investment. 



The muscles connecting the stem-joints one with another, and 

 the various ossicles of each cirrus are also formed from mesenchyme 

 cells ; and the same thing is true of the muscles connecting the arm- 

 joints with one another. No embryological distinction can be 

 perceived between the cells forming the dorsal elastic ligaments of 

 the arm-joints, which straighten the arms, and the ventral flexor 

 muscles which bend them, and we are confronted with the remark- 

 able fact that nervous, muscular, connective, and skeletal tissues 

 arise from cells of an identical origin. 



Meanwhile the portion of the axial organ which is situated in 

 the calyx has been growing. It becomes plexiform, its original 

 single tube-like cavity being split into several cavities. Where its 

 originally lower end reaches the wall of the stomach it gives off five 

 branches, at first solid but soon becoming hollowed out, one of which 

 grows into each arm and there forms the genital rachis. 



According to Perrier, the septum dividing the oral and aboral 

 sections of the coelom contained in the arm is a secondary formation, 

 and has nothing to do with the original septum dividing oral and 

 aboral coeloms from one another. This may be doubted, and a 

 re-examination of this point would be desirable. The finished 

 genital organ is nothing but the apex of the branch of the rachis 

 which penetrates the pinnule. As in the case of other Echinoderms, 

 the duct is produced by an outgrowth of the cells of the genital 

 organ itself, which burrow their way to the outside. 



By the secondary thickening of the radial plates, the primary 

 ampullae of the chambered organ become completely overarched and 

 shut off from the aboral coelom. The under-basals unite with the 

 uppermost stem ossicle to form the centro-dorsal ossicle, on which 

 numerous new cirri make their appearance. This centro-dorsal ossicle 

 grows till it reaches the primary radials and shuts off the basals 

 from the exterior. These unite to form a single piece, the rosette 

 plate, which forms the roof of the chambered organ. 



The young Crinoid in this stage, in which arms, pinnules, and cirri 

 are developed, is termed a Pentacpinoid larva, from its resemblance 

 to the genus Pentacrinus. 



Soon after this the young Crinoid wrenches itself loose from the 

 stalk, which parts just below the centro-dorsal, and so enters on its 



