ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 155 



protractor. The rest are inserted into the edges of a thin plate, the 

 " tongue-plate," which is closely applied to the whole of the upper and 

 a part of the anterior lower surface of the cartilages, being as it were 

 bent over their anterior extremities. The applied surfaces of the plate 

 and of the cartilages are perfectly smooth, so that the former can play 

 readily over the latter, like a rope upon its pullc}'. The upper surface 

 of the tongue-plate carries in the middle a single row of tridentate 

 teeth ; outside these is a row of conical spines and broad flat-edged 

 plates, and most externally there are one or more rows of recurved 

 hooks, which, when the organ is at rest, lie over and nearly meet one 

 another in the middle line. 



When this organ is in action, it is commonly more or less protruded 

 from the mouth by the protractor muscles ; the large lateral spines of 

 the tongue are divaricated (giving rise to that resemblance to the oral 

 armature of Sagitta which has been remarked), so as to get all the 

 teeth to bear ; and then by the alternate action of the upper and lower 

 sets of muscles inserted into the tongue-plate, a chain saw-like move- 

 ment is communicated thereto, in consequence of which the teeth act 

 as a rasp or saw upon any body with which they are brought in 

 contact. 



The buccal cartilages take no part in the movement of the tongue- 

 plate, but simply act as its pulley. 



The QLsophagus widens so gradually into the stomach, that no dis- 

 tinct line of demarcation can be drawn between the two, but the latter 

 narrows suddenly into the intestine at a short distance in front of the 

 " nucleus." 



The Liver (/) is composed of several foliaceous masses containing- 

 many oil-globules ; it opens by a wide duct into the angle of junction 

 of the intestine and rectum. 



The Circulatory System (figs, i, 2, 3, 6). — This consists of a very 

 perfect heart with an aorta and its branches, but there is no trace of 

 any venous system. 



The heart lies anterior to and parallel with the rectum ; its axis, 

 therefore, is nearly at right angles with that of the body. It possesses 

 two chambers, an auricle (?/) and a ventricle (?'). The auricle is large 

 and somewhat elongated, extending above into the elevation which 

 carries the subspiral ciliated band. Its wall is formed by branched 

 and interlacing muscular fibres, which are attached partly to the 

 parietes, partly to the walls of a contractile sac (c), to be mentioned 

 presently. 



The ventricle is almost globular, and has thicker walls, in which the 

 separate muscular fibres are not distinguishable. The lips of the 



