128 COMPARATIVE ANATOMY CHAP. 



The retractor capitis lateralis (2), which is also paired, rises from 

 the same point as the depressor infundibuli, runs into the head, and 

 is attached to the cephalic cartilage. The retractor eapitis medianus 

 (3), originally paired, but usually become single by fusion, arises at 

 the posterior (inner) side of the shell, and also runs into the head, 

 and is attached to the cephalic cartilage. 



In the Dibranchia, the first muscles which fuse are the two median retractors of 

 the head (Onychoteuthis), these then fuse more completely with the lateral retractors 

 (Ommastrephes, Sepioteuthis, Loligo, Sepiola), so that finally (Sepia) the whole of the 

 musculature running from the shell into the head forms a muscular sheath open 

 posteriorly. This sheath encloses the lower portion of the visceral cavity, which is 

 principally occupied by the digestive gland or liver, and thus forms a kind of 

 muscular hepatic capsule. The posterior opening in this capsule may finally become 

 completely closed by the depressor infundibuli, in that, on the one hand, its 

 anterior edges fuse with the posterior and median edges of the capsule, and, on the 

 other, it sends out numerous muscles to the diaphragm, forming the diaphragma 

 musculare. 



The muscular hepatic capsule, i.e. all the muscles forming it, the 

 retractors of the head and the depressors of the siphon, may without 

 doubt be accepted as the homologue of the columellar muscle of other 

 Molluscs. Like the latter, they run down from the shell or its vicinity 

 into the head and foot (represented by the siphon). 



The adductors of the funnel (5) have still to be mentioned. 

 They rise from the cephalic cartilage and run upwards and backwards 

 to the funnel. Finally, the collaris (4) is a strong muscle which runs 

 forwards right and left from the wall of the funnel, and is attached 

 to the lateral edges of the nuchal cartilage. In the Octopoda and Sepiola, 

 where a pallio- nuchal concrescence (cf. pp. 54, 55) has rendered a 

 nuchal locking cartilage unnecessary, the collaris passes uninterruptedly 

 over the neck like a saddle, forming a closed circle round the nuchal 

 portion of the body. 



XIII. The Nervous System. 



(As a general introduction to this section the reader may be referred to pp. 27, 28.) 



A. Amphineura. 



The nervous system of the Amphineura is very significant from 

 the point of view of the comparative anatomist. Its most important 

 peculiarities may be briefly described as follows : 



1. The ganglionie cells are either not at all or not exclusively 

 localised in definite ganglia. 



2. Four nerve cords run through the body from before backward. 

 These contain not only nerve fibres, but ganglion cells distributed along 

 their whole length. They might suitably be called medullary cords, 

 and must be considered as belonging to the central nervous system. 



