XVI THE VOYAGE OF H.M.S. CHALLENGER. 



embedded in a fissure, varying in shape in different species, and are reduced in size and 

 modified in form, the capacity of vision being confined to two small points, one on the 

 upper or dorsal, and the other on the lower or frontal surface (PI. XIII. fig. a), and in 

 each it exists only to a limited extent, the organ being without the power of movement. 

 Yet in the young, as may be assumed from the appearance of the embryo of Willemcesia 

 (PI. XX. fig. 2), as observed previous to the escape of the brephalos from the ovum, the 

 ophthalmopoda are globular in form, and distinctly pedunculated. 



It is interesting to find in the same geological epoch some specimens of Eryon 

 that are blind, and others with large and probably well-developed organs of vision. 

 But it is not more remarkable than that living and adult specimens of Cambarus should 

 be found with the ophthalmopoda in all stages of development, from the well-formed 

 eyes of those that live in the waters of America that are open to the sun, to the blind 

 forms dwelling in subterranean caves where light never penetrates. The loss of vision 

 is not necessarily a disadvantage to a species, while its surroundings, both in relation to 

 food and companionship with others necessary for its existence, are convenient, since sight 

 is useless where there is no light and the absence of the organs of vision may lessen the 

 risk to life while the conditions are permanent, but should these be withdrawn or vary, 

 the want of sight must be detrimental in the struggle for existence and thus be a pre- 

 lude to the extinction of a species. 



The species of the Eryoniche live at the bottom, where their food is abundant ; for 

 in the ancient as well as in the modern seas the myriads of organic forms constantly 

 falling to the bottom from the extensive area of waters above in which they live, con- 

 stitutes a continuous and unfailing supply of food that comes within their reach ; and 

 thus organs of vision are not necessary for the purpose 'of seeking food. Thus the 

 Eryonidse live and renew their species under conditions where other forms might 

 perish. 



I have previously remarked that the ophthalmopoda in the Eryonidse are depreciated 

 in character ; but it should also be noticed that the departure in the recent genera takes 

 place, as shown in PI. XIII. a, in a direction that resembles that seen in Benthesicymus 

 and Gennadas among the Dendrobranchiata (PI. LVII. fig. la, and PL LIX. fig. 1). 

 This is a point of considerable interest, since it is the only instance in which the 

 secondary eye or ocellus is observable in the Trichobranchiata. In the division Dendro- 

 branchiata the ophthalmopod is generally compressed, and the ophthalmus possesses 

 a reniform shape, which in some species has the margin on the upper and inner 

 surface projecting somewhat beyond its limit and forming an imperfect ocellus, or 

 small secondary visual organs, so situated that it is capable of being useful as an 

 organ of vision when the animal otherwise is at rest. 



The ophthalmopoda in most of the species are projected at the extremities of a 

 narrow transverse rod that probably represents the ophthalmic somite, and is sometimes 



