SPECIAL ORGANS. 93 



Tunicata. With Ascidians eight pigment- or eye-specks 

 (ocelli) have been detected at the entrance of the respiratory 

 tube; and six of a deep yellow color at the entrance of the 

 anal tube. (Owen.) 



Lamellibranchiata. Eyes often numerous. Each possesses 

 a highly refractive crystalline lens, and a pigment lining 

 (choroid coat) forming an iris anteriorly. They may be sit- 

 uated along the margin of mantle, or, as in burrowing forms, 

 around the orifices of the breathing tubes. 



Gasteropoda. Eyes, when present, two in number, situ- 

 ated at the extremities of tentacles in fresh- water and pulmo- 

 nate forms, at the outer side of base in marine. In the 

 latter case they are fixed, but in the former can be protruded 

 and protracted at will. A conspicuous optic nerve traverses 

 each tentacle. The crystalline lens often lenticular, and vit- 

 reous humor viscid. Eye particularly conspicuous in Strombus, 

 Natica, and Bulla. Janihina is without eyes. In Strombus a 

 sclerotic coat, choroid, iris, crystalline lens, and cornea are 

 present. 



Cephalopoda. The Tetrabranchiata, as in Nautilus, have 

 small sub-hemispherical eyes placed upon pedicles, and are 

 probably without crystalline lens. In Dibranchiata, as in 

 Sepia, the eye is singularly complex. Within an orbit formed 

 in great part by the integument, the bottom of the cavity 

 alone being cartilaginous, is placed the eyeball covered ante- 

 riorly by the skin. This is very thin and transparent where 

 it extends in front of the eye, to assume the function of a 

 cornea. The sclerotic coat is incomplete in front, and per- 

 forate behind through numerous openings for the passage of 

 branches of the optic nerve. The second tunic, the retina, 

 is thick, and extends forward in a remarkable manner to the 

 anterior portion of the eye where it appears to hold the crys- 

 talline lens in position. The pigment layer (choroid coat) is 

 apparently placed within the retinal layer. In this anomalous 

 position it is din^c^tlxTunderstand how rays of light can pass 

 through the pigment layer to be received upon the optic ex- 

 pansion. With some cuttlefishes a granular nerve-like layer 

 has been recognized within the pigmental layer which has 



