THE SKULLS OF FISHES. 191 



but far behind the .anterior end of the notochord, the myelon, 

 dilating very slightly, suddenly terminates, and with it, the 

 neural canal. The lateral muscles are divided into segments 

 corresponding with the pairs of spinal nerves, and the most 

 anterior of these segments is situated just behind the slightly 

 dilated chamber of the neural canal which contains the cor- 

 respondingly enlarged end of the nervous axis. The latter is all 

 that represents the brain, and the chamber is the skull. 



A ciliated sac placed at b, in connection with the upper 

 surface of the brain, has been considered to be the olfactory 

 organ of this fish, but it is possible the sac may simply represent 

 the pineal body ; optic nerves (b) are given off to the rudimen- 

 tary eyes, and the branches (c) appear to be analogous in function 

 to the fifth pair. But no pituitary body has been recognised, 

 and, what is still more singular, there is no trace of auditory 

 sacs. A cartilaginous ring, provided with tentacular prolonga- 

 tions (/, g), surrounds the mouth, and there is a singular 

 branchial skeleton more like that of an Ascidian than any 

 ordinary vertebrate structure ; but neither of these structures 

 probably have anything to do with the true cranial or facial 

 skeleton. 



It will be observed that this very remarkable skull, if it can 

 be properly so called, is not strictly comparable to an arrest of 

 development of a higher vertebrate skull ; the notochord ex- 

 tending far beyond the end of the cranium, which it never does 

 in any embryonic condition of a higher Vertebrate. 



B. The Cartilaginous Cranium. 



Of this there are three forms : in the first (a) there is no 

 mandible ; in the second (b) the mandible is present, and the 

 suspensory apparatus by which it is connected with the skull 

 forms one mass with the latter ; in the third (c) the mandible 

 is also present, but the suspensory apparatus by which it is con- 

 nected with the skull is freely moveable. 



a. The cartilaginous cranium without a mandible. 



This kind of cartilaginous cranium is found only among 

 the Marsij)obranchii, or Lampreys and Hags, and a descrip- 



