80 ECHTNODERMATA. 



down the sides of the lantern, to pass through the auricles and 

 up the ambulacral tracts, to the ocular plates. They can easily 

 be seen along the sides of the test, but are difficult to see be- 

 fore they leave the lantern. 



3. Along the course of each radial canal, the ampullce, which 

 supply the tube feet, are to be seen. The relations of the tube 

 feet and radial canals are practically the same as in the starfish 

 except that the removal of the radial tubes to the inner sides 

 of the ambulacral plates causes two perforations for each foot 

 here, while the starfish has only one. One of these perforations 

 is for the connection between the ampulla and the foot, the other 

 is for the connecting tube between the radial canal and the foot. 

 The connecting tube joins the foot outside of the plate (as in 

 the starfish), while it joins the radial canal inside of the plates 

 (different from the starfish). 



Remove the intestine and study the lantern and its attach- 

 ments. 



1. The whole lantern is inclosed in a delicate membrane, 

 the periphanjngeal or lantern membrane which contains the 

 lantern ccelom. This space communicates with the five radial 

 perihemal canals, which run along the ambulacral areas be- 

 tween the radial canals and radial nerves, and with the dermal 

 branchse. It is important in respiration. 



2. The tip of the lantern is attached to the flexible peristoma, 

 and muscles extending from various parts of it are attached to 

 the hard parts of the surrounding test. 



In shape the lantern is a five-sided radially symmetrical 

 pyramid. Each of the sides consists of a massive calcareous 

 structure, the alveolus, which supports an elongated tooth the 

 tip of which projects through the peristome. The base of the 

 pyramid may be compared with a wheel, in which the ten 



Gemmill, J. F.: The Locomotor Function of the Lantern in Echinus with 

 Observations of the Allied Lantern Activities. Proceedings Royal Soc. 

 of London, vol. No. 85, 1912. 



