342 COMPAEATIVE AXATOMY. 



aud auditory organs, as well as the point of origin of a largo 

 number of muscles. In Nautilus this cephahc cartilage is formed 

 of two pieces, united along the middle line, and drawn out into 

 anterior as well as posterior processes; these surround the com- 

 mencement of the oesophagus. In the Dibranchiata the cephalic 

 cartilage is much better developed. It consists of a median portion, 

 ■which is traversed by the oesophagus (Fig. 179, K), and of two 

 lateral processes, which are sometimes mere flat enlargements, in 

 which case accessory cartilaginous plates are added on to them to 

 form the orbits ; at other times they are more highly developed, 

 are then continuous with superior processes [K'), and completely 

 enclose the orbits. The central nervous system ((?) is placed on 

 that portion of the cephalic cartilage which is traversed by the 

 oesophagus. 



The Dibranchiata are provided with additional cartilaginous 

 skeletal pieces. A dorsal cartilage is the most common. In the 

 Sepiadfe this forms a semi-lunar piece, which lies in the anterior 

 dorsal region of the mantle, and is continued into two small lateral 

 comua ; in Octopus, where there is no median pieces, we find the 

 cornua only. 



There is a cartilaginous plate in the neck also, in addition to 

 two cartilages at the base of the funnel — the hinge-cartilages. 

 These are less constantly present than those which lie at the base of 

 the fins, and which are found in all Dibranchiata provided with fins, 

 for they serve as the point of attachment for the muscles of these 

 organs. 



Muscular System. 

 § 265. 



We can understand how it is that separate groups of muscles 

 are so feebly developed, if we bear in mind that there is a dermo- 

 muscular tube united with the integument and external organs of 

 support, and that these, notwithstanding the great modifications 

 which they undergo, have very much the same character in all cases. 

 With this may be correlated the absence of internal organs of 

 support in the lower divisions, and their relatively slight develop- 

 ment in the higher ones. The muscular system is made up of band- 

 like fibres, which, not unfrequently, give indications of their greater 

 differentiation by the possession of transverse strife. 



In the LameUibranchiata the adductor muscles, which pass either 

 transversely or obliquely through the body from one valve to the 

 other, are those that are best developed. There are either two of 

 them, which form bundles separated by some distance from one 

 another, one anterior (Fig. 167, m a), and the other posterior (vu i^), 

 as in Unio or Ajiodonta; or there is but one muscle, which corre- 

 sponds to the hinder one of the Dimyaria, and occupies the middle of 



