180 ROBERT TRACY JACKSON ON ECHINI. 



The muscles of the lantern are very complex. They have not been described in adult 

 Strongylocentrotus, though Loven (1892) showed them in a very young one and in exquisite 

 detail in the adult of several types. There are sixty muscles in Strongylocentrotus and other 

 regular Echini. There are ten protractors (Plate 5, figs. 1, 4, pr.; text-fig. 229), which take 

 origin on the upper outer face of the half-pyramids and epiphyses and are inserted at the base 

 of the two half-interambulacra which are associated with the intervening ambulacrum (compare 

 text-figs. 218, 219, p. 191). The protractors serve to extend the mouthparts, which the animal 

 actively does when in good condition. The ten retractors (Plate 5, figs. 1, 4, re.), which serve 

 to open the jaws, take origin on the lower outer face of the half-pyramids and are inserted on 

 the auricles. The retractors of the two half-pyramids that are united by the interpyramidal 

 muscle pass to the two auricles of the same ambulacrum (compare text-fig. 219, p. 191). Ten 

 radial compass muscles take origin from the bifurcated termini of the compasses and are in- 

 serted at the base of the two next adjacent half-interambulacral areas. From studying their 

 motions in life, and from their position and wide divergence, it seems that the function of the 

 radial compass muscles is to maintain and restore the vertical position of the lantern. When 

 alive, the mouth with the peristome is freely and actively extended and retracted, and not in 

 a vertical plane only, but with much sidewise or inclined motion, so that the radial compass 

 muscles are in just the position to effect the motion and restore the perpendicular. It is note- 

 worthy that in clypeastroids, where the lantern is in almost immediate contact with the base of 

 the test and little motion is possible, the protractor muscles are very small and the compasses 

 with their muscles are absent. 



The distribution of protractor, retractor, and radial compass muscles is immediately con- 

 nected with the view expressed (pp. G2, 190), that the corona may be considered as made up 

 of five rather than ten areas. Five large but short interpyramidal muscles (Plate 5, fig. 2, ip.), 

 extend from the corrugated lateral wing of each half-pyramid to the similar wing of the next 

 adjacent half-pyramid. These muscles serve to close the jaws and from their size must be very 

 powerful. Ten tiny internal brace muscles (Plate 5, fig. 12, i. b.) extend from the brace to 

 the two associated epiphyses, and ten similar external brace muscles, (e. b.), extend also from 

 the brace to the epiphj^ses, binding these parts together and doubtless giving some mobility. 

 Five circular compass muscles (Plate 5, fig. 1, i.) extend between the compasses, being inserted 

 at the proximal end of the outer of the two pieces of which each compass is composed. Their 

 function is not evident. 



In brief the sixty lantern muscles of Strongylocentrotus and other regular Echini 

 are as follows: ten protractors; ten retractors; five interpyramidal muscles; ten internal 

 brace muscles; ten external brace muscles; five circular compass muscles; ten radial compass 

 muscles. 



Such being the character of the lantern of the adult, it is important to see what is the 



