338 NATURAL SCIENCE [May 



' stereom ' forms by far the greater part of the whole animal ; even 

 the walls of the theca may be so strongly calcified that one marvels 

 how the creature lived with so little space in which to pack its 

 nerve-centre, stomach, and other vital organs. Some of the early 

 cystid ancestors of the crinoids had no stem or arms, but merely a 

 theca like an irregularly plated sac, affording little information as to 

 the structure of the animal beyond the position of mouth, anus, 

 water-pore, and generative opening. But in the crinoid, concomi- 

 tantly with the development of stem and arms, a more regular 

 arrangement of the thecal plates arose, and the various skeletal 

 elements became more intimately connected with the organs of the 

 body. It is not difficult for us, by comparison with recent forms, 

 to get a fair idea of the internal anatomy of the most ancient 

 crinoids. We can predicate the course of the nerves or the intimate 

 structure of the tissues connecting the plates in any extinct type of 

 crinoid, with no less correctness than the anatomist of Yertebrata 

 can infer the position of the muscles in an Eocene ungulate ; and it 

 is doubtful whether our conclusions would be modified in any im- 

 portant point had the subjects of our study been preserved to us by 

 the most approved laboratory methods instead of as petrifactions. 



The cup, in its simplest form, consists of two circlets of five 

 plates. Each plate of the upper circlet supports an arm and is 

 called a ' radial ' ; the plates of the lower circlet, the ' basals,' rest 

 on the stem and alternate with those of the upper circlet, i.e., are 

 inter-radial in position. Some crinoids have yet another circlet 

 below these, and the constituent plates are called ' infrabasals ' ; 

 they are radially situate. The tegmen in most primitive forms, as 

 well as in the embryonic stages of the living Antedon, consists of 

 five large triangular plates, alternating with the radials, and called 



* orals,' because they roof over the mouth. Between one of the orals 

 and the two adjoining radials there usually opens the anus, while 

 this same posterior oral is pierced by one or more water-pores. The 

 arms consist of a series of ossicles, called ' Ijrachials ' ; each is joined 

 to the radial by a muscular articulation, permitting of motion up and 

 down, while each brachial is usually united to its successor by a 

 similar articulation. The arms usually fork more than once, and the 

 brachials after the radial and after each successive forking are termed 

 brachials of the first order, of the second order, and so forth, or, 

 as I have found more convenient, ' primibrachs,' ' secundibrachs,' 



* tertibrachs,' etc. There passes along each arm, forking with it, an 

 extension from each of the chief systems of the body ; and to receive 

 these the brachials are grooved on the ventral surface. We need 

 only note here the axial nerve passing from the dorsal nerve-centre 

 (' chambered organ ') and innervating the arm-muscles ; and the 

 ciliated food-groove which sends a continuous current of water down 



