108 



of the Crinoid with its longer axis vertical and the upper end just under 

 the centre of the ventral disc. Its lower extremity approaches but does 

 not quite touch the bottom of the. visceral cavity. Its walls are composed 

 of minute polygonal plates or of an extremely delicate network of anas- 

 tomosing fibres. The five ambulacral canals are attached to the upper 

 extremity, radiate outward to the walls of the cup and are seen to pass 

 through the ambulacral orifices outward into the grooves of the arms. 

 (Am. Jour. Sci. vol. xlviii, p. 31.) 



The ambulacral canals of the Crinoidea are, for the greater part, 

 respiratory in their function. They are, however, as most naturalists 

 who have studied their structure will admit, truly the 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 pro- 

 vided 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 greatly in form and number in the difi'erent genera. That 

 of the Starfish Asteracanthion rubens is thus described by Prof E. 

 Forbes. " 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 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 

 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 cross-fish." 

 (Forbes, British Starfishes, p. 73.) 



In Prof. Joh. Muller's work, " Uber den bau der Echinodermen," 

 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 consisting of a minute tissue of calcareous 

 fibres, or of small polygonal plates. The walls are also sometimes 

 minutely poriferous. In all the Holothurians the madreporic organ 

 is a sack attached by one of its ends to the oesophageal canal, the 



