REPORT ON THE ISOPODA. 153 



family to which it belongs, but in the Isopoda generally; it is true that in certain 

 forms (e.g., Phryxus) degraded by parasitism, there is a simdarity between the 

 abdominal appendages, but such a case does not interfere with the truth of the 

 general statement that has just been made, which only refers to perfectly developed 

 forms. 



There is an interesting analogy between the structure of Anuropus, as regards its 

 respiratory organs, and that of a deep-sea Isopod lately described by Professor A. Milne- 

 Edwards. 1 



In Bathynomus giganteus — a colossal Isopod measuring 9 inches in length — the 

 respiratory organs are very remarkable and unlike anything that has been met with in 

 other Isopoda except in Cepon and lone. " It appears," says M. Milne-Edwards, " that 

 the respiratory apparatus of an ordinary Isopod is insufficient to fulfd the physiological 

 needs of Bathynomus, and that the development of special organs of a greater functional 

 power has been rendered necessary. The abdominal limbs, which ordinarily in this 

 group constitute the sole branchial apparatus, form in Bathynomus a kind of oper- 

 cular system, beneath which are found the real respiratory organs or branchiae." 

 These structures are, in fact, represented by a series of branched outgrowths of the 

 abdominal wall, which contain abundant blood spaces, as has been proved by injection. 

 The branched appendages of the genera Cepon and lone are quite rudimentary as com- 

 pared with those of Bathynomus. 



Now, in Anuropus the same need for increased respiratory power has been satisfied in 

 a different way. Instead of the development of accessory branchial organs, as in Bathy- 

 nomus, an additional pair of abdominal appendages have been pressed into service as 

 gills. Anuropus is, therefore, more typically Isopodan in structure than Bathynomus, 

 and, indeed, presents us with an exaggeration of a character which is common to the 

 group, and forms an important item in its definition. 



It is evident from these two instances that there is a need for increased respiratory 

 surface in deep-sea animals, and, not to go into a detailed summary of facts, I may 

 remind the reader that the dorsal processes of some of the curious deep-sea Holothurians 

 described by Dr. Hjabnar Theel are probably analogous. It is well known that the 

 percentage of oxygen is less in the bottom waters than at the surface, and hence 

 has arisen the need to make the most of the failing supply of this gas. I have 

 carefully examined the other Isopoda with a view to "discover if there were present 

 any other modifications of a like nature to those which occur in Bathynomus and 

 Anuropus. In a deep-sea Sphseromid described above under the name of Cymodocea 

 abyssorum, the branchial organs are more fully developed than in the majority of species 

 belonging to that family. In the typical Sphseroniidse, as is well known, only the fourth 



1 Comptes rendus, January 1879. There is a figure (two-thirds the natural size) of this most remarkable form in a 

 work by M. Filhol, lately published, and entitled La vie au fond des mers ; see p. 148 for description. 



(ZOOL. CHALL. ESP, — PART XLVIII. — 1886.) Bbb 20 



