NARRATIVE OF THE CRULSK. 



173 



found from the littoral zone clown to the lowest points reached by the dredge. In the 

 different zones these genera may present modifications ; for instance, the Amphiurce, below 

 1000 fathoms, often have more numerous mouth papillae, and the corresponding Ophio- 

 glyphcB usually have swollen arm-plates and a microscopically tuberculous surface. Such 

 structural features, however, plainly have no connection with the conditions of life, nor have 

 they any relation to the survival of specially favoured forms. From a depth of over 

 1500 fathoms are found the strongly armoured Ophiomusium pulchellum, (see fig. 

 71), the delicate Amphilepis, and the Ophiomitra chelys (see fig. 72), with its thorny spines 

 and soft disk. At that great depth the peculiar conditions, apparently so unfavourable to 

 a rich and varied growth, have not checked the development of widely differing forms. 



" While, however, the Ophiuroidea yield little to the dictation of light, heat, or water 

 pressure, they show well-marked laws of 

 growth. Certain genera take the lead, like 

 the larger clans of a barbarous nation. The 

 collections of the Challenger, when combined 

 with those of the " Blake," show that the 

 four genera Ophioylypha, Amphiura, Ophia- 

 cantha, and Ophiothrix contain more than 

 two-fifths of the known species. There is 

 a tendency also to elaboration and variety 



structure. The naked and embryonic 



in 



Fio. 72. — Ophiomitra chelys (Wyv. Thorns.). Dorsal aspect 

 of tlie disk ; four times the natural size. 



genera, like Ophiomyxa and Ophioyeron, 

 have few representatives ; while the finely 

 constructed Ophioglyplia has many species, 

 and even the highest group, composed of 

 the closely allied Ophiura, Pectinnra, and 

 Ophiopeza, is pretty strong in numbers. 



" The dredgings of the Challenger have 

 further taught us that we must not look exclusively in the abysses for surprising- 

 shapes, or for those that connect us closely with geological times. If the singular 

 Ophiotholia (see fig. 73) must be sought in 1800 fathoms, its relative Ophiohelus may be 

 found in less than 1 00 fathoms ; and if Ophiomastus from the deep sea brings to mind 

 the extinct Aspidura, Pectinura of the littoral zone recalls the" so-called Ophiura of 

 the Oolite. Nor must we forget that the extraordinary Astrophiura 1 , apparently 

 intermediate between the Brittle-stars and the Starfishes, lives in shallow water." 



The " singular Ophiotholia" above referred to, was discovered by Mr. Murray in the 



1 Sladen, On the structure of Astropliiura, a new and aberrant genus of Echinoclermata, Ann. and Maij. Nat. Hist., 

 ser. 5, vol. jv. pp. 401-415, 1879. 



