1869.] BILLINGS — ON THE STRUCTURE OF CRINOIDEA, ETC. 431 



homologues of those of the Echinodermata in general. In the 

 higher orders of this class the canals are usually more specialized 

 than they are in the lower — being provided with prehensive or 

 locomotive organs. In all of the existing orders, including the 

 recent Crinoidea, we find an oesophageal ring. 



To this organ, which is only a continuation of the canals, are 

 attached the madreporic appendages. These consist of small 

 sacks, or slender tubes, varying greally in form and number in 

 the different genera. That of the Starfish Asteracanthiou ruhens 

 is thus described by Prof. E. Forbes : — " On the dorsal suiface 

 is seen a wart-like striated body, placed laterally between two of 

 the rays ; this is the madreporiform tuhercle or nucleus. When 

 the animal is cut open, there is seen a curved calcar< ous column 

 running obliquely from the tubercle to the plates surrounding the 

 meuth; Dr. Sharpey says it opens by a narrow orifice into the 

 circular vessel. It is connected by a membraae with one side of 

 the animal, and is itself invested with a pretty strong skin, which 

 is covered with vibratile cilia. Its form is that of a plate rolled 

 in at the margins till they meet. It feels gritty, as if full of sand. 

 When we examine it with the microscope we find it to consist of 

 minute calcareous plates, which are united into plates or joints, so 

 that when the investing membrane is removed it has the appear- 

 ance of a jointed column. Prof Ehrenberg remarked the former 

 structure, and Dr. Sharpey the latter: they are both riL'ht. Both 

 structures may be seen in the column of the common cross-fish." — 

 (Forbes, British Starfishes, p. 73.) 



In Prof. Joh. Mailer's work, " Uber den bau der Echinoder- 

 men," several forms of the madreporic appendages of the different 

 groups of the recent Echinodermata are described. In general 

 they are composed of a soft or moderately hard skin, consistinir 

 of a minute tissue of calcareous fibres, or of small polygonal 

 plates. The walls are also, sometimes, minutely poriferous. In 

 all the Holothuiians the madreporic organ is a sack attached by 

 one of its ends to the oeiophageal canal, the other extremity 

 hanging freely down into the perivisceral cavity, not connected 

 with the opposite body wall as is the sand canal of the starfishes. 

 (Op. cit., p. 84.) In its consisting of a convoluted plate the 

 madreporic organ of Actiiwcriuus, therefore, agrees with that of 

 the starfishes, while in its being only attached at one extremity it 

 resembles that of the Holothurians. 



The convoluted plate of the Palaeozoic Crinoids and the madre- 



