192 LECTURE X. 



In the aberrant Sipunculoidea the tube-feet are wanting, together 

 with the calcareous corpuscles, the worm-like form and annulation 

 of the integument are more decided, but the chylaqueous fluid con- 

 tinues to be agitated in the abdominal cavity by the cilia of the lining 

 membrane. 



Before commencing the demonstration of the anatomy of the 

 Asteroidea, I may point out the chief characters of the allied order 

 Crinoidea, in which the radiated disc is fixed by a long jointed 

 calcareous stem to some foreign body, as they are shown in the 

 existing species, called Pentacrinus Caput-Medusce, the type of a 

 very numerous assemblage of analogous pedunculated star-fishes, 

 which existed in countless myriads during some of the ancient 

 (secondary) periods of geology. Those species in which the stem is 

 cirrigerous and pentagonal, as in the recent form, are called "Penta- 

 crinites : " those in which it is rounded, and seems to have been 

 devoid of cirri, are called " Encrinites." Both kinds have received 

 the common name of " stone-lilies." Their remains sometimes con- 

 stitute extensive tracts of marble-limestone. The Melocrines are 

 more especially the crinoid prototypes of the Stelleridce. 



The stem of a stone-lily is composed of numerous joints or segments 

 having a central aperture, which, when separated, are called " wheel- 

 stones," or " entrochi : " casts of their cavity remaining after the 

 calcareous wall 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 the margin of the 

 cup proceed five jointed arms, which radiate and divide into delicate 

 ten taenia. The upper side of the arms support numerous short 

 jointed cirri or pinnules. Groups of five long and slender cirri 

 radiate at nearly equidistant points from the stem of the recent 

 species of Pentacrinus. Both the arms and pinnules are grooved 

 along their ventral side, which groove is bridged over by a membrane 

 called "perisome." 



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 con- 

 dition of their digestive system the Pentacrinites and Comatulfc 

 correspond with the fettered Bryozoa among the polypes. The free 

 Comatula is a step in advance, and manifests its affinity to the 



* CLVIII. p. 295. 



