Name the principal Sense- Organs of the Craniata. 

 The chief organs of special sense are : 



f 1. The organ of vision or eye. Paired and 

 brain-derived. 



2. The olfactory organ or nose. Single or 



paired. 



3. The auditory organ, otocyst or ear. 



Paired. The functions are balancing 

 and hearing. 



4. The lateral line canal (of Fishes and 



some Amphibians) in skin along the 

 body and branching upon the head ; 

 represents a series of sense-buds for 

 " detecting slow vibrations in the 

 water, a function closely allied to 

 hearing." The sensory jelly-tubes 

 (of Elasmobranchs) in the snout and 

 skin of ventral surface of body ; 

 these have ampullae (containing 

 sensory cells) at the inner ends, and 

 the outer ends are porous. 



5. Tactile corpuscles of skin, vibrissae or 



tactile hairs of face, and taste-buds 

 of the tongue papillae of Mammals. 



Origin from 

 sense-cells 

 of skin. 



How is the Eye developed ? Describe its Structure. 



The side wall of the thalamencephalcn develops a rounded 

 optic outgrowth, which is or becomes hollow and which 

 narrows about the base to form the short optic stalk (optic 

 nerve, ultimately) ; and it comes into contact with the 

 inner surface of 'the ectoderm, which there thickens (lens 

 rudiment) and pushes it in, causing it to become invaginated, 

 a two-layered optic cup. A fissure (closed later) occurs in 

 the cup- wall ventrally, and is continued as a groove along 

 the optic stalk. The outer part of the cup- wall becomes 

 the thin pigment layer of the retina ; the inner part becomes 

 the retina, and it secretes the clear jelly (vitreous body) 

 which fills the cavity of the cup. 



The hollow lens is detached from the ectoderm ; later it 

 is solid and crystalline. An ingrowth of mesenchyme 

 into the vitreous body, via the fissure, forms either a vascular 

 tissue, the pecten (in Birds and Reptiles), or a muscular 



