G. 0. SÅRS. 



[No. 1. 



of the shell. The muscles performing the above-mentioned move- 

 ments, are very small and not easy to examine. 



Movements of the Antennæ. — The antennæ being, as above 

 stated, the only natatory organs of the animal, their movements 

 are very powerful and somewhat complicated. At the base, 

 however, these organs are råtner firmly connected with the skin 

 of the head, and will admit of scarcely any movement at this point. 

 The chief mobility of these organs is in the scape itself, the 

 distal part of which, together with the branches, is moved as 

 a whole on the proximal part of the scape. This movement 

 rendered possible by the great flexibility of the scape, is chiefly 

 accomplished by the above-described strong muscles that run from 

 the upper part of the head down to the antennæ and partly 

 atfix themselves to the middle of the scape, partly conti- 

 nue to the bases of the branches. By the contraction of these 

 muscles the distal part of the scape becomes more or less 

 strongly curved outwards, and at the same time the branches are 

 spread out from each other so as to fully expose their densely 

 ciliated natatory setæ, thus exhibiting a rather wide surface for 

 striking the water. On moving back, the branches approach 

 each other. and their setæ become lowered along the sides of 

 the joints, so as to render the least possible resistance to the 

 water. During the swimming motion, these movements are rather 

 energetic, and recur at very short intervals, thus propelling the 

 animal through the water at quite an even speed. A very similar 

 movement of the antennæ is also found to take place while 

 the animal is creeping on the bottom, but in that case these 

 organs are moved very much slower, so as easily to admit of 

 a study of the several phases of the movement. It is then 

 found, that also the branches themselves perform a movement of 

 their own, being at times curved very strongly outwards, so as 

 nearly to touch the lateral surfaces of the shell. 



Movements of the Labrnm. — By the aid of the two above- 

 described slender muscles running obliquely downwards to the base 

 of the labrum, this part admits of being lowered, or of being removed 

 from the other oral parts so as nearly to form a right angle 



