l888.] NEW-YORK MICROSCOPICAL SOCIETY. 5. 



proboscis-like mouth. The ribbon, firmly attached to a strong 

 bed, or subradular membrane, rests loosely upon, and passes 

 over and under the anterior end of a cartilaginous apparatus, 

 named the "skeleton." 



Huxley's description of the skeleton is that it is "composed of 

 two principal masses of partially fibrous, or completely cartilagi- 

 nous tissue, odontophoral cartilages, which may be more or less 

 confluent, and are further united together in the middle line by 

 fibrous and muscular tissue. Their anterior ends and oral 

 faces are free and smooth, and are usually excavated so as to 

 present a trough-like surface to the subradular membrane, 

 which rests upon them." 



In Sycotypus this skeleton has the form of an attenuated trun- 

 cated triangle. The " odontophoral cartilages " forming the 

 longitudinal borders of the triangle, are connected by a tough, 

 transversely striated substance, and they each present along their 

 entire upper surface a distinct, smooth, white trough for the free 

 action of the subradular membrane. The radula, with its strong 

 bed, rests upon this cartilaginous triangle, and its forward por- 

 tion is bent over and underneath the truncated end of the tri- 

 angle, in order to accomplish its intended work, which we will 

 notice presently. The radula, when at rest and especially as re- 

 gards the posterior portion, has the form of a tube, slit along the 

 entire upper surface, causing the points of the teeth of this tubu- 

 lar portion to converge towards the axis of the tube. But where 

 the anterior portion is flattened out, the teeth lie in transverse 

 rows, with the points directed backwards ; and where the radula 

 curves over the forward end of the skeleton the teeth necessar- 

 ily raise their points, nearly perpendicularly to the surface of the 

 membrane, giving the appearance of a very formidable weapon. 

 The radular membrane is continued posteriorly in the form of a 

 white, opaque, closed tube, constituting the radular sack. And 

 the radula itself is the product of growth from the inner surface 

 of this radular sack. As the teeth and the membrane are worn 

 away by use at the anterior end, new teeth come into service, 

 and the whole apparatus is preserved in its effectiveness by the 

 growth of the radula at its posterior end, and its advancement 

 along its entire length. The line of origin of the teeth in the 

 radular sack is quite sharply defined, and the radula easily sepa- 

 rates from the sack at this line of origin. 



