114 LECTURE X. 



foramina, through which a multitude of small tubes or hollow ten- 

 tacula can be protruded and retracted, and these constitute the 

 common organs of adhesion and locomotion in the Echinoderms. 



Before commencing the demonstration of the principal characters 

 of the Stelleridm, or Star-fishes, I may observe that in one existing 

 species of an allied family (^Crinoidece), the radiated disc is fixed by 

 a long jointed stem to some foreign body, as you perceive in this 

 Pentacrinus Caput Medusa, the type of a very numerous assemblage 

 of analogous pedunculated star-fishes, which existed in countless my- 

 riads during some of the ancient periods of geology : their remains 

 sometimes constitute extensive tracts of marble-limestone, and are 

 known by the names of Stone-lilies, or Encrinites. 



The stem is composed of numerous joints or segments having a 

 central aperture, which, when insulated, are called wheel-stones, or 

 "Entrochi:" casts of their cavity remaining after the calcareous 

 walls have been dissolved away constitute the " screw-stones " of the 

 Derbyshire chert, and other transition limestones. The jointed column 

 supports at its summit a series of plates forming a cup-like body, 

 containing the viscera, and from whose upper rim proceed five jointed 

 arms, which radiate and divide into delicate tentacula. The upper 

 side of the arms support numerous short jointed cirri. Groups of five 

 long and slender cirri radiate at nearly equidistant points from the 

 stem of the recent species. 



The form of star-fish to which the radiated capital of the crinoideal 

 column bears most resemblance is that which is presented by the 

 species of Comatula, the ova of which have been discovered by 

 Dr. V. Thompson to pass through a pedunculated pentacrinite state, 

 before their final metamorphosis into a free star-fish. 



In the condition of their digestive system, the Pentacrinites and 

 Comatulfe correspond with the Bryozoa among the polypes. The 

 Pentacrinus may be regarded as a gigantic form of pedunculated 

 Bryozoon. The free Comatula is a step in advance, and manifests 

 its affinity to the gelatinous Radiaria by its mode of swimming : 

 the movements of its pinnate arms exactly resemble the alternating 

 stroke given by the Medusa to the liquid element, and with the same 

 effect of raising the animal from the bottom, and propelling it back 

 foremost. 



The rays of the ordinary star-fishes are not cirrigerous or bifur- 

 cated : their soft external integument is supported by a tough co- 

 riaceous membrane, strengthened by calcareous matter disposed in 

 a coarsely reticulate form upon the dorsal and lateral aspects of the 

 radiated body, and arranged in series of more compact and regu- 

 larly-formed transverse pieces, which bound each side of a lon- 

 gitudinal furrow, extending along the under surface of each ray 



