16 



in the progressive motions of the animal, attest an obvious tendency towards 

 the Gasteropoda. And while tracing these examples of affinity with the different 

 and heretofore widely separated groups oi Mollusca, between which this remark- 

 able Form, 1 apprehend, is osculant, there may also be perceived in the whole of 

 this singular but at the same time regular and spnmetrical arrangement of 

 palpigerous organs about the mouth, an analogical relation to the higher An- 



nulosa. 



§ 2. Of the Muscular System. 



Before describing the Muscular System, it becomes necessary to notice the 

 internal skeleton or framework (fig. 1. pi. 8.) from which its principal masses 

 take their origin. Like that of the Dibranchiate Cephalopods, this skeleton is 

 cartilaginous, yields readily to the knife, and in texture and semitransparency 

 closely resembles the cartilage which constitutes the skeleton of the Skate. In 

 Sepia this cartilaginous part completely encircles the oesophagus, and on the 

 dorsal aspect of that tube is dilated into a large cavity, which contains the 

 brain ; but in Nautilus the circle is incomplete behind, and the brain is pro- 

 tected only by its membranous sheath. 



The central mass of the cartilage, or body of the skeleton {a. fig. 1. pi. 8.), is 

 situated on the ventral aspect of the oesophagus : it is of a triangular form, with 

 its base towards the oesophagus, and the dorsal angles are produced on each side 

 of that tube as far as the optic ganglions. These angles, which may be termed 

 the cephalic pi'ocesses (6. b. fig. 1. pi. 8.), have a deep semicircular groove 

 anteriorly (c. fig. 1. pi. 8.) for the lodgement of the optic ganghons and part of 

 the nervous coUar surrounding the oesophagus. From the anterior part of the 

 body of the cartilage two other processes {d. d. fig. 1. pi. 8.) arise, which di- 

 verge and pass forwards within the crura of the funnel for about half an inch, 

 and terminate in those lateral projections at the sides of the funnel which appear 

 jvist above the aperture in the mantle. Behind the origin of these processes the 

 caitilage is continued, in the form of a ridge, for a little way between the great 

 muscles of the shell. 



In the body of the skeleton is excavated a large sinus, which receives the con- 

 tents of the veins of the head and funnel, and empties itself into the commence- 

 ment of the great dorsal vein. 



The muscular fibres of the great oral sheath arise from the whole of the 



