Royal Microscopical Society. 55 



IV. — Note on the Calcareous Parts of the Sucking Feet of an 

 Echinus (Podophora atrata). 



By Charles Stewart, M.E.C.S., F.L.S. 



(Read hefore the Kotal Microscopical Society, Jan. 1, 1873.) 



Plate VII. 



In addition to the spines and pedicellaria which cover the outer 

 surface of an Echinus, or Sea-urchin, five rows of tubular muscular 

 cylinders may readily be seen ; these are called the ambulacral 

 tubes, and are the sucking feet which assist the animal in loco- 

 motion. The feet are most developed on the under surface of the 

 Echinus, those on the upper surface being commonly modified, and 

 used either as organs of touch or for respiration. 



In all known Echini, these feet are strengthened by a calcareous 

 framework, which consists always of two, and usually of three, 

 distinct parts, each composed of many separate pieces. Two of 

 these, called the rosette and ring, are apparently constructed for 

 keeping the sucking extremity expanded ; the third exists in the 

 form of numerous spicula, which vary greatly in shape in the dif- 

 ferent genera, and afford valuable aids for their distinction. 



The rosette is usually figured and described as a fiat circular 

 disk composed of separate pieces, varying in number from three to 

 six or seven, five, however, being the most usual ; its margins are 

 ornamented by spine-like projections, and its centre is perforated by 

 a large hole. The ring is described as a single polygonal plate, 

 having a large hole in the centre, whose sides are equal in number 

 to the plates of the rosette. 



This description, however, is not quite true, either for the 

 rosette or ring. 



Although the general outline of the rosette is correct, the mar- 

 ginal spines do not stand out horizontally, but project forwards 

 from the fiat anterior surface ; this arrangement seems to enable 

 them to grasp the body to which the sucker is applied when its 

 centre is pulled towards the animal. The part of the rosette nearest 

 the animal is very complex, each plate showing a depression of 

 great depth, so that the inner as well as the outer margin of the 

 rosette is thin, the greatest thickness being found a little to the 

 inner side of the centre of each plate. Just inside the outer margin 

 of the depressed area may be seen a small extremely transparent 

 tubercle ; it is upon this tubercle that the angles of the polygonal 

 ring rest, and to the constant motion of the ring upon the rosette 

 must be attributed the solidity and consequent transparency of the 

 tubercle. The posterior contiguous edges of the plates of the 

 rosette are bevelled off, so that their anterior diameter is greater 

 than their posterior ; the result of this is, that when the posterior 



