414 Mr. E. Billings on the Structure of 



nucleus. When the animal is cut open, there is seen a curved 

 calcareous column running obliquely from the tubercle to the 

 plates suiTOunding the mouth ; Dr. Sharpey says it opens by 

 a narrow orifice into the circular vessel. It is connected by a 

 membrane 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 gi'itty, as if full of sand. When we exa- 

 mine 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. Professor Ehrenberg remarked the 

 former structure. Dr. Sharpey the latter : they are both right. 

 Both structures may be seen in the column of the common 

 cross-fish." (Forbes, ' British Starfishes,' p. 73.) 



In Prof, Joh. Miiller's work, ' Ueber den Bau der Echino- 

 dermcn,' several forms of the madi-eporic appendages of the 

 different groups of the recent Echinodermata are described. 

 In general they are composed of a soft or moderately hard 

 skin consisting of a minute tissue of calcareous fibres or of 

 small polygonal plates. The walls are also sometimes mi- 

 nutely poriferous. In all the Holothurians the madreporic 

 organ is a sac attached by one of its ends to the oesophageal 

 canal, the other extremity hanging freely down into the peri- 

 visceral cavity, not connected with the opposite body-wall, as 

 is the sand-canal of the starfishes [oj). cit. p. 84). In its con- 

 sisting of a convoluted plate, the madreporic organ of Actino- 

 crimis 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 paleozoic Crinoids and the 

 madreporic sacs and tubes (or sand-canals) of the recent 

 Echinoderms, therefore, all agree in the following respects : — 



1. They have the same general structure. 



2. They are all appendages of the ambulacral system. 



3. They are all attached to the same part of the system — 

 that is to say, to the central point from which the canals 

 radiate. 



The above seems to me sufficient to make out at least a good 

 prima facte case for tlie position I have assumed. When 

 among the petrified remains of an extinct animal we find an 

 organ which has the same general form and structure as has 

 one that occurs in an existing species of the same zoological 

 group, we may, with much probability of being correct in our 

 opinion, conclude that the two are homologous, even although 

 we may not be able positively to see how that of the fossil is 



