NATURAL HISTORY 45 



geotropic are: Psammechinus microtuberculatus (Baglioni, 1905) and 

 Psammechinus miliaris (Bolin, 1926; Lindahl and Runnstrom, 1929). 



The blastulae and gastrulae oi Arbacia punctulata swim at the surface 

 of the water; the plutei are scattered (Lyon, 1906b, 1907). The sur- 

 face swimming is not due to Hght for it occurs in total darkness, nor 

 is it due to oxygen supply for it takes place when the tube containing 

 them is inverted. This is a true negative geotropism since the larvae 

 are heavier than sea water; their specific gravity is about 1.06 (Lyon, 

 1906b, 1907). The blastulae from centrifuged eggs also come to the top, 

 though the heavy pigment makes them heavier on one side (Lyon). 



g. Locomotion 



Sea urchins move in any direction by means of their spines and tube 

 feet (Holmes, 191 2). According to A. Agassiz (1872-1874, p. 264) "The 

 mode of moving of Arbacia is quite different from that of our common 

 Strongylocentrotus ; instead of dragging itself along by means of the 

 suckers of the actinal surface, it makes free use of its spines, and by a 

 sort of tilting motion advances quite rapidly. The spathiform shape of 

 the spines around the actinosome in species of this genus is undoubted- 

 ly due to the wear and tear produced by this means of locomotion." 



Specimens of Arbacia punctulata have been observed to move over the 

 glass of an aquarium at the rate of 35-40 mm. per minute after being 

 disturbed; they average 22.2 mm. per minute when undisturbed 

 (H. W.Jackson, 1939). I observed a very small Arbacia, 6 mm. in dia- 

 meter (with spines) move at the rate of 3 mm. per minute. 



Echinus sphaera ( esculentus ) and lividus (probably Paracentrotus lividus) 

 moved along a horizontal surface 6 in. per minute, and up on a vertical^ 

 surface i in. in 4 minutes (Romanes and Ewart, 1881 ; Romanes, 1885). 

 Gemmill ( 191 2) found that Echinus esculentus and miliaris usually moved, 

 out of water, i in. in 5 minutes, the fastest being 3-4 in. in 5 minutes. 



Lytechinus variegatus ascended a glass plate with an average speed of 

 1.8 mm. per minute, and a maximum speed of 12 mm.; on a vertical 

 surface it progressed by means of its tube feet exclusively. On a hori- 

 zontal surface of sand it progressed by means of its spines exclusively 

 at the rate of 82 mm. per minute, average, with a maximum of 137 

 mm. (Parker, 1936). 



The sand dollar, E'cAznflracAmM^j&armfl, averaged 13.7 mm. perminute, 

 with a maximum of 18 mm., when progressing in the sand (Parker, 



1927)- 

 As early as 1712, the locomotion of sea urchins aroused the attention 



