164 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



There are between 350 and 400 raised tubercles on the ribs, 

 which tubercles are somewhat flattened and hatchet-shaped on the 

 surface. They are studded front and back with the same crystalline 

 pavement. It is not difficult to assign a reason for this 

 arrangement. The eyes having no movement of their own are 

 located in every position to command a prospect all around. 

 Direct vertical vision is secured by the eyes on the surface, and 

 horizontal vision at every angle from the slopes and sides of tho 

 tubercle. 



The nacre of Trigonia is transparent in all the species, but it is. 

 so abundantly permeated with nerve-tissue as to be opaque for 

 microscopic purposes except in the thinnest sections. I believe 

 that the colour of the nerve-tissue which appears so dark and 

 black in sections, is purplish with a greenish iridescence, and this 

 is seen on the inner lining of most of the shells ; but the purplish 

 tint is also visible on the outer plate or tegmentum. 



The eyes have a distinct pupil and are easily examined in section.. 

 Though the lenses are singularly bright and lying close together, 

 yet there are interstices with micropores upon them. There are- 

 smaller tubes seen in amongst those carrying the optic nerves and 

 crossing . them at right angles. The office of these I have not 

 satisfactorily made out. The eye-capsules it may be added, are 

 very easily examined, but I cannot be sure that all carry that 

 peculiar knob and an eye with lens, retina and optic nerve of 

 the invertebrate type, because in making the sections the utmost 

 caution could not prevent the disappearance or displacement of 

 those minute and delicate organs. 



Now what is the meaning of this wonderful aggregation of* 

 visual organs ; or is there anything like it to be found in nature T 

 Nothing equal to it, I think we may say with certainty. The 

 only things that can be likened to the valves of Trigonia are the 

 Ommatidia of the insect world. Those who are not familiar with 

 the technology which I have already explained, at least know 

 what is meant by the brown, spherical eyes on the head of the 

 too-familiar house-fly. All of us know it and most of us have 

 seen the hundreds or thousands of hexagonal facets under a lens. 

 In the head of the common drone-fly there are said to be between 

 4,000 and 5,000 eye-facets. Usually in the shell-eyes of Mollusca 

 they occupy a much larger space, but here they do not, and I 

 therefore ask my readers to estimate the difference in size between 

 a drone-fly's head and the valve of a Trigonia; and then take into 

 account what must be the powers of vision of these prodigies, 

 amongst the sight-seers. 



Now considering that the eyes are thus on the surface of the 

 two valves of the shell, is it a very great stretch of hypothesis to 

 regard the two valves as the representatives of the sides of the 



