288 ANIMAL PHYSIOLOGY. [LESSON 89. 



1289. That the combination of these tissues is necessary to the 

 production of vision, is evidenced by the fact that they are all, or 

 nearly all, found in various stages of modification, down to animals 

 of otherwise lowly organization. 



1290. There are, however, many animals altogether destitute of 

 eyes, and yet they are visibly affected by light. This is especially 

 well seen in the Animalcules, Zoophytes, and others, where some of 

 them (Hydra) seek the light, and others (Actinia, and many other 

 Zoophytes) contract their bodies, and shun its influence. 



1291. Plants, guided by light, open or close their flowers and 

 their leaves, and follow with flowers expanded the daily course of the 

 sun, or seek his light with branches and leaves slowly moving in the 

 direction he takes ; yet they possess no nervous system, neither do 

 the animals above mentioned, but both appear to be influenced by 

 perceptions altogether unknown to us. 



1292. Some authors (Ehrenberg and others) have supposed that 

 many animals, acknowledged to be destitute of a nervous system, 

 possess eyes ; thus red spots seen in many (supposed) animalcules, to 

 say nothing of other lowly organized creatures, have been described 

 by Ehrenberg, as visual organs, notwithstanding that many of these 

 have subsequently been discovered to be plants ! 



1293. This red coloring-matter, so prominently developed in the 

 lowest plants, as in the plant called "fied snow;" the Protococcus 

 pluvialis, found in rain-water ; the Hcematococcus sanguined, found 

 frequently on stale bread ; is but a slightly altered chlorophylle, and 

 into which it can be converted, made green. 



1294. These facts clearly demonstrate the fallacy of attributing 

 the function of vision to the red spots, unless supported by more 

 convincing testimony than mere color. ' Eyes sometimes have a red- 

 dish tint from the red-brown pigment which shows through the 

 transparent cornea. 



1295. The possession of eyes in the Radiate sub-kingdom, appears 

 to be open to much doubt, notwithstanding that visual organs have 

 been claimed for the Medusae, Starfishes, &c. 



1296. In the Articulata, on the contrary, there is no question 

 of their very general development, although they are not found in 

 the Epizoa, generally, nor in the Cirripeds, at all. 



