54 bulletin: museum of comparative zoology. 



The nuclei are large and spheroidal, measuring, in some cases, as much as 

 0.008 nun. in diameter. The nuclei in the more superficial parts of the organ 

 are much larger than those situated more deeply. 



In the centre of the thicker part of the cone are a number of scale-like 

 bodies, placed at riglit angles to its axis (c. s.). There are from ten to twelve 

 of these scales, measuring on an average 0.08 mm. in diameter, and 0.012 mm. 

 in thickness. In structure they resemble, on the whole, those forming the 

 posterior cup. They stain in the same way and to the same depth, but they 

 are, on the whole, somewhat thinner in proportion to their diameter, and the 

 fibres composing them are more obvious. 



The form of this central mass constitutes the most important difference 

 between the present species and P. margaritifera, for in the latter it is much 

 shorter, the conical part only e.xtending halfway along the siiles of the inner 

 cup, and being hollowed to receive the deeper portion of a structure I liave 

 called in that species the "internal cone." 



5. The Anterior Cap (a. c.) covers the whole surface of the organ above the 

 central mass. It has the form of a meniscus, the deeper surface, which rests 

 upon the central mass, the inner cup, and the edge of the posterior cup, being 

 deeply concave, whilst the ui)per surface is very convex. The greater part of 

 it is composed of delicate wavy fibres interlacing with each other, and disposed 

 in general parallel with the surface of the organ. Over the central mass the 

 fibres are closer and more nearly parallel than over the lateral jiortions of the 

 organ, where the texture is much more open, leaving rounded interspaces be- 

 tween them. A few nuclei were seen in these lateral parts, but none in the 

 centre. 



In the superficial layers of this anterior cap are a number of scales disposed 

 parallel to the surface. They very closely resemble the scales of the outer 

 layers of the posterior cup, but are somewhat larger in diameter and not quite 

 80 thick. They stain very deeply, and the more superficial ones are almost 

 homogeneous in composition. There are a few of these scales in the deeper 

 layers, but these are much smaller and show a tendency to break up into fibres 

 something like the scales in the middle of the central niass. 



II. The Sipho.val Organs (Fig. 1, S. S.) are paired and lie just within the 

 hinder margin of the funnel, and are seen peeping out from beneath it when 

 the mfintle-cavity is opened. Eacli organ is bluntly pyriform in shape, or 

 more accurately, perhaps, has the form of an acorn in its cup (Fig. 5). The 

 smaller end is directeil backward.-*, and at the inner side of the broad end is 

 a kind of stalk by which it is attached to the ventral wall of the body. The 

 free distal end is slightly pigmented, while the broad end is paler and the tip 

 is a paler patch of a semi-transparent yellowish horn color. The length of 

 the organ is about 0.9 nun. and the greatest diameter 0.8 mm. 



In minute structure the siphonal organs clo.^cjy re.>«emble the ocular organs 

 in their main features (Fig. 0). It will be sutlicient therefore to eniimerate 

 their connlitiu'.nt parts and to mention the pt>inta in which tiiey present dill'er- 

 enccs. On the whole, ihcy are more like the organs of P. margaritifera. 



