214 THE YOYAGE OF H.M.S. CHALLENGER. 



fathoms. Most of the abyssal species are blind, and the eyes have undergone a process 

 of degeneration which is tolerably uniform in all. The eye-stalk is frequently prolonged 

 in the form of a short spine, while the visual portion is pale in colour and absolutely 

 devoid of pigment. A distinct corneal surface is always present, though in one species 

 {MunidojJsis pilosa, Henderson) it is remarkably reduced ; and in nearly all cases the 

 degenerative changes have taken place without any marked reduction in the size of the 

 eye as a whole. Some of the blind species are remarkable for the great length of their 

 antennge, rendering it probable that the loss of sight is partially compensated by an 

 increased development of the tactile sense. The puzzling fact that certain deep-water 

 species are blind, while in others belonging to the same group and found at similar 

 depths the eyes are well developed, has been frequently commented on, and the explana- 

 tion that the former have probably migrated into deep water at an earlier period, and 

 have consequently had sufficient time to undergo modification, appears to be the most 

 satisfactory one. The blind Galatheids share so many features in common, and the most 

 widely separated types are so frequently connected by intermediate forms, that the 

 retention of certain of the genera which have been founded for their reception must, I 

 hold, be regarded as questionable. Since the return of the expedition other naturalists 

 have instituted five new genera, all of which are represented in the Challenger 

 collection. 



A very conspicuous feature is the prevalence of species of Munida. They are found 

 almost everywhere in deep water, though but few reach a depth of 1000 fathoms, which 

 appears to represent their bathymetrical limit. Prior to the Challenger investigations 

 not more than half a dozen species were known to science, but recent deep-sea dredgings 

 have increased the number to upwards of thirty ; no less than fifteen of the species in 

 the present collection are described as new. In none of them — with a single exception 

 — do we meet with any striking modification, though in most cases the eyes are slightly 

 enlarged, a feature commonly observed in those deep-sea animals in which the visual 

 organs are still functional. The exception referred to is that of a species named Munida 

 microphthcdma by A. Milne-Edwards in his Preliminary Eeport on the " Blake " Crustacea, 

 which was taken by the Challenger in both the Atlantic and the Pacific. In this 

 species signs of commencing degeneration are apparent, the eyes being remarkably 

 reduced in size, and the cornese of a light brown colour. 



In the majority of the deep-water Galatheids — with the exception of those belonging 

 to the genus Munida — the eggs carried by the female are few in numl^er and of 

 remarkably large size. It may be inferred from this that in the deep sea enemies are 

 fewer, and the chances of each individual egg undei'going its full development therefore 

 relatively greater, the result being diminished production on the part of the parents ; 

 while the large size of the ova perhaps indicates a protracted embryonic existence. 



Two well-marked genera, Ptychogaster and Uroptychus, and to a lesser extent a third. 



