The Locomotor Function of the Lantern in Echinus. 85 



requires to be supplemented in one important particular (p. 91), as well as 

 to be extended under certain conditions to locomotion under water. 



In 1910, Miss Abel, a student attending one of the Nature Study Classes 

 at the Millport Marine Station, noticed that the specimen I had set her to 

 watch seemed to move by lurches occurring at intervals, and that just prior 

 to each lurch the whole urchin rose up slightly from the table surface. I 

 have pleasure in acknowledging this observation as the starting point of the 

 following paper. 



The fact was verified in Miss Abel's specimen as well as in several others, 

 and it was seen that the rising, prior to each lurch, was effected by protrusion 

 of the lantern of Aristotle, the tips of the five teeth being brought together at 

 a certain stage in the movement, and forming a powerful central stilt on 

 which the greater part of the weight of the urchin for the time being rested. 

 It was surmised that, besides lifting up the urchin, the muscular apparatus 

 of the lantern also pushed it forward at each lurch, the place where the teeth 

 rested on the table serving as the fulcrum or fixed point in the movement. A 

 record of the direction and extent of the movements of the first urchin was 

 then made by setting it to travel over a large sheet of paper, and making a 

 dot on the paper immediately below a marked spine, at the end of every 

 lurch. This record is reproduced in fig. 1. Nothing further could be done at 

 the time, but during last spring and summer I had the opportunity of 

 examining the question further with the help of the plentiful material 

 available from the tanks at the Millport Station. It is perhaps an indication 

 of the healthy character of these tanks that urchins which had been in them 

 for three weeks or so, travelled just as vigorously as others which had been 

 brought in on the actual day of the experiment. The observations have 

 reference primarily to Echinus esculentus L., but apply also in their essentials 

 to Echinus miliaris (Gmel.), allowance being made for differences in size, length 

 of spines, etc. 



Description of Action. — When an urchin is travelling freely (p. 93) out of 

 water, there occurs a rhythmic swinging movement of the lantern in the 

 direction of progression. The movement is made up of a backward* and 

 a forward stroke. The former is the pushing or poling stroke and is 

 accompanied by protrusion. The latter is the return stroke, serving to bring 

 the teeth forward again into position, and it is naturally accompanied by 

 lifting or retraction of the lantern. In detail, the sequence of events is as 

 follows, for urchins of small or medium size {e.g. up to 3| inches in 

 equatorial shell diameter) : — Just after a lurch has been accomplished the 



* The terms forward, backward, anterior, posterior, lateral, have reference here to the 

 direction of progression. 



