198 LECTURE X. 



fluid to the vesicles at the base of the hollow tentacles protruded 

 through the ambulacral pores. Tiedemann and Sharpey * also agree 

 in rejecting the continuation of the chylaqueous or erectile system of 

 the feet from the intestinal vascular system. In the Crinoids a cordi- 

 form sac lies at the bottom of the cup-shaped body, from which vessels 

 diverge along the axial canal of the arms, pinnules, and cirri, and a 

 vessel descends into the spongious axis of the body-cavity and stem. 

 A chylaqueous canal, which runs immediately beneath the tentacular 

 furrow, conveys the fluid to the hollow feet : in the Comatula the 

 canal is divided by a medi-vertical septum. In the Ophiurid(E two 

 or four broad respiratory lamellce project from each of the five 

 inter-radial angles into the visceral interspace. 



In some star-fishes there is a small membranous tube, filled with 

 calcareous particles, called by Tiedemann the sand-canal : its position 

 is indicated by the circular multiperforate prominence or nucleus 

 {Jig^ 95, z), on the dorsal aspect of the disc of the Asterias, near the 

 angle between two of the rays, which prominence resembles a minia- 

 ture brain- stone madrepore. In other Stelleridce a jointed calcareous 

 column {fig. 95, s\ is continued from the nucleus into the interior of 

 the body, and consists of minute hexagonal plates, which are united 

 into larger joints. The precise function of these appendages to the 

 nucleus is not yet understood. From the analogy of one of its modi- 

 fications with the jointed column of the crinoid star-fishes, it has 

 been suggested that it may be the analogue or remnant of that 

 column ; but, according to the observations of M. Sars, the Asterise 

 are not fixed animals in the young state. Dr. Sharpey has con- 

 jectured that it may serve as a filter in the admission of sea-water to 

 the tubular system of the ambulacral feet : and it unquestionably 

 relates to the chylaqueous system, of which it is a part distinguished 

 by the calcareous deposits superadded to the canal. The sand-canal 

 is adherent to the parietes of the body in the OphiuridcB and 

 EchinidcB as well in the Stelleridce : but the spot to which it adheres 

 in the Ophiuridce is imperforate. In the Holothuriadce it hangs 

 loose in the abdominal cavity. In the Asterophyton the madreporoid 

 nucleus is situated on the ventral aspect of the disc. 



As the sea-water is freely admitted into the general cavity of the 

 body, and bathes all the viscera, their vascular surfaces thus stand 

 in the relation of a respiratory organ to the aerated medium, and they 

 are every where provided with vibratile cilia, which maintain the 

 currents of oxygenated fluid. f But with the sea-water is mingled a 

 kind of chyle-corpuscles, long since recognised by Erdl J, the forms of 

 which in difierent Echinoderms have been lately described by 



* CLX. p. 35. t CLXT. X CLXII. p. 58. 



