22 MORPHOLOGICAL TYPES 



Head. — The outstanding advance which the lamprey shows 

 over the condition of Amphioxus is the possession of a definite 

 head. A head is a specialisation of the anterior region of the 

 body brought about in connexion with : 



the development of paired sense-organs for perception 



at a distance ; 

 the correlated specialisation of the nerve-tube into a 



brain. 



To these is added in higher forms the specialisation of the 

 most anterior gill-bars into jaws for the capture of food ; but 

 this need not be considered here as the lamprey has no jaws. 

 The organs of the head are protected by a special skeletal 

 structure called the skull. 



Sense-Organs. — The nose consists of two sacs invaginated 

 from the skin, and whose epithelium is specialised for the 

 perception and detection of chemical substances. This 

 epithelium sends nerve-fibres to the brain, forming the 

 olfactory nerves. While the paired nature of the olfactory 

 organ is easily seen on dissection, it is outwardly obscured by 

 the great expansion and upgrowth on each side of the region 

 corresponding to the upper lip. This modification causes 

 both the nasal pits and the hypophysial sac to open to the 

 outside by a single common median dorsal pore. 



The main rudiments of the paired eyes in development 

 grow out from the brain on each side, giving rise to the optic 

 vesicles. The outer side of each vesicle is pushed in so as to 

 convert it into a cup, and the lens (developed from the super- 

 ficial skin) fits into the mouth of the cup, just beneath its rim. 

 The inner layer of the cup contains the cells which are sensitive 

 to light, and form the retina ; the outer layer forms a backing 

 of pigment. Outside this again, two mesodermal layers are 

 laid on. The innermost of these is the choroid which contains 

 blood-vessels, the outer, which is also the outermost of the 

 whole eyeball, is the hard and protective sclerotic. The 

 sclerotic encloses the whole eyeball, but in front of the lens it 

 is transparent, forming the cornea. The cornea is in contact 

 with the epidermis, which is here also transparent, forming the 



