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bodies, which will be described hereafter. The under 

 surface of the body presents the mouth in the centre, and 

 deep grooves radiate from it to the extremities of the arms. 

 These grooves contain the suckers, curious tubular exten- 

 sile organs, filled with a fluid, and capable of adhering to 

 the surfaces of bodies by means of a terminal disk. They 

 serve as feet, and by means of them the animal can walk 

 about with great facility. They are arranged in longi- 

 tudinal rows. On the dorsal surface is seen a wart-like 

 striated body placed laterally between two of the rays : 

 this is the madreporiform tubercle or nucleus. When the 

 animal is cut open, there is seen a curved calcareous 

 column running obliquely from the tubercle to the plates 

 surrounding 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 in- 

 vested 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 hexagonal calcareous plates, which are 

 united into larger plates or joints, so that when the in- 

 vesting membrane is removed it has the appearance 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 

 Crossfish. The latter distinguished anatomist, speaking 

 of the probable uses which have been conjectured as the 

 purposes of this singular organ, says, " Tiedemann con- 

 ceives the function of the sand canal (the term he 

 applies to it) to be that of secreting the earthy matter 

 required for the growth of the calcareous skeleton. Mi- 

 cheli considers this view as very improbable ; — and the 



