ORGANS OF THE SENSES. 251 



gans of sense, and none have been detected in the lowest cystic 

 forms of this class. In the scolex, however, among the ces- 

 toid entozoa, two minute red-coloured shining eyes have 

 been long observed, and two of a dark colour are found be- 

 hind the mouth in the polystoma, one of the flat trematoid 

 worms. But these organs are more commonly perceived on 

 those remarkable entomoid kinds of parasitic worms which 

 attach themselves to the surface of aquatic animals, and are 

 in that situation more exposed to the influence of light and 

 of the surrounding element, as various forms of lernsese. 

 The eyes are sessile in these epizoa, and most commonly 

 form a single organ on the median plain, as in many of the 

 entomostracous Crustacea which they at first so much re- 

 semble. In some the eyes are more numerous, and are 

 placed apart from each other, as in the gyrodactylus auricu- 

 latus, found attached to the gills of the bream, where there 

 are four minute red-coloured eyes, placed in two pairs be- 

 hind each other on the back part of the head. In the ach- 

 theres percarum, there is a single round prominent eye on 

 the fore part of the head, between the two short antennae, 

 which is very obvious through the coverings of the ovum, 

 and in the young animal in its free unattached condition, and 

 in this species the organ of vision remains through the whole of 

 life. But in the lernceocera cypinacea, where the little red 

 coloured circular eye, placed on the fore part of the head, 

 is also distinctly seen in the ovum, and continues through- 

 out the free and entomostracous state of this remarkable 

 animal, it is entirely lost after the metamorphosis and no 

 trace of it can be detected in the adult fixed condition of 

 this parasitic worm. The same disappearance of the eyes 

 has been observed in some of the rotifera. In nearly all the 

 rotiferous animalcules, however, eyes are distinctly observ- 

 able, of a round form, of a red colour, often two in number, 

 sometimes united to form a single organ, sometimes four as 

 in the meglotrocha, or a greater number as in the cydoglena, 

 and placed on the upper and fore part of the body. Their 

 constancy x their number, and their high development in the 

 wheel-animalcules corresponds with the general advanced 

 organization and the numerous active organs of locomotion 

 possessed by these minute transparent animals. From the 

 transparency of all their parts and the deep red colour of the 



