406 Baldwin Spencer: 



central cavity. In the specimen drawn in Figure 3, the process 

 is hor&e-shoe shaped, which is probably its normal form. It is 

 very small — its relative size can be judged from the figure re- 

 ferred to, which is five times the size of the original. The one 

 represented in Figure 9 is a part of the structure in another 

 specimen that was stained and cut by the freezing microtome. 

 In this the close part of the horse-shoe extended round one 

 quarter of the oral opening, that is, each of the two limbs cor- 

 responded in position to one of the cellular bands on the wall of 

 the central cavity. Unfortunately, the section was somewhat 

 broken, and I am unable, amongst the broken parts, to deter- 

 mine definitely the nature of one of the limbs, which appears to 

 be somewhat smaller than the one figured, which has retained 

 its original position. In one of the four specimens in which 

 this organ can be seen, the part attached to the oral surface is 

 alone present, the two limbs of the horse-shoe either not having 

 been developed, or, more likely still, they have been knocked 

 off by the buffeting of the waves on the shore. 



The general structure is seen in Figure 9, which represents, 

 as the section was a thick one, a drawing of the solid object. 

 The part attached to the oral .surface has the appearance of a 

 semifibrous gelatinous band containing many nuclei, often ar- 

 ranged in rows (d). It is apparently attached along its whole 

 length, but in parts there are remarkable lines of nuclei (e) 

 associated with structures that look like special attachments to 

 the jelly around the mouth. When examining this section under 

 the dissecting miscroscope, I separated the greater part of the 

 horseshoe lying between the two limbs from the oral margin 

 with comparative ease, but at each of the points from which the 

 limbs depend the attachment is a very firm one. It will be 

 noticed that the nuclei, if such they be, of the bands with which 

 this horseshoe-shaped organ is connected are much larger than 

 those of the latter. 



The limb shows three different parts, first a fibrous part (a), 

 second a gelatinous ]Kirt with regularly arranged nuclei (b). and 

 third, a well-marked row of densely packed nuclei (c), lying 

 along a well-marked track between the first and second. At the 

 spot marked N, the limb has a decidedly narrow neck where it is 

 attached to the edge of the oral opening (o.m."). In this region 



