THE CH^TOGNATHA, OR PRIMITIVE MOLLUSCA. 369 



The occurrence of three pairs of supporting plates in con- 

 nection with the spines on the surface of tlie buccal mass is 

 an important feature in Chastognath anatomy, but of far wider 

 significance is the complicated chitinous (?) skeletal piece 

 with its large lamella and two lateral wings serving for the 

 attachment of the retractor, extensor, and other muscles in 

 the liead. This internal cephalic skeleton we believe to be 

 the homologue of the cartilaginous cephalic skeleton of the 

 Cephalopoda, and bears a superficial resemblance to that of 

 Nautilus (text-fig. 5). Its presence in addition to the many 

 other points of resemblance between the two groups should 

 be something more than a mere case of analogy. 



Nervous System. 



The remarkable resemblance of the ChEetognatli and 

 Molluscan nervous systems was recognised as long ngo as 

 1844 by Krohn, the discoverer of the nervous system in 

 Sagitta hexaptera, and again by Limgerhans, who dis- 

 covered the buccal ganglia. The two most recent accounts 

 by Hertwig and Gourret are not entirely in agreement, but 

 both may be readily referred to the Molluscan plan. Indeed, 

 the resemblance is so close that, in a description of the 

 Ch^tognath nervous system, the Molluscan terminology may 

 be appropriately employed. 



Certain characteristics which are primitive among the 

 MoUusca are retained in the nervous system of the Chasto- 

 gnatha, but these co-exist with others which appear to be 

 directly associated with the more active mode of life of the 

 latter. In two features, namely, in the widely distributed 

 nerve plexus which enwraps the body, and in the close 

 adhesion of the central nervous system to the ectoderm, the 

 Chgetognatha are more primitive than the Mollusca; the 

 ladder-like arrangement of the nerves that occurs in 

 Amphineura, for instance, is not a Chgetognath feature, 

 although the nerves issuing from the ventral ganglion are 



