732 ON A NEW ISOPODAN GENUS (FAMILY ONISCIDAe), 



from their first and second pieopods, the others may also be 

 males, but I cannot feel certain without dissecting the pieopods, 

 and as they appear quite similar in all other characters, it 

 seemed hardly necessary to do this. 



Remarks. — The occurrence of this Oniscid living in the waters 

 of Lake Corangamite raises an interesting question to which it 

 is not easy to give a decisive answer. All the other members of 

 the Oniscidae known to me are more or less strictly terrestrial. 

 Some of them may be found near the seashore, but they do not 

 live actually in the water, but in places under stones, leaves, 

 etc., where the air is moist. Most of them it is true are still 

 branchial breathers, but the outer branches of their pieopods 

 form opercula v &nd are pressed close to the body enclosing the 

 branchiae in spaces where the air can be kept moist for long 

 periods of time;* in other cases the branchiae themselves are 

 specially modified to allow of their breathing air or they 

 even contain air passages and cavities that act as tracheae f. 

 In Haloniscus searlei on the contrary the outer branches of the 

 pleopoda are particularly large and delicate and hang loosely 

 below the body and they perhaps act as branchiae supplement- 

 ing the branchiae proper formed by the inner branches which 

 are smaller in comparison than is usual in the family. 



There is however no doubt that the Oniscidae and the other 

 terrestrial Isopoda are originally descended from marine an- 

 cestors. Some such as Scyphax, Scyphoniscus and Deto,X be- 

 longing to the family Scyphacidae closely allied to the Onis- 

 cidae, still live on the seashore between tide marks and are 

 consequently periodically submerged. Their mouth parts, pleo- 

 poda, etc., are on the whole of a slightly more primitive charac- 

 ter and not so specialised as in the more strictly terrestrial 

 Oniscidae under which Haloniscus searlei must be placed. The 



•Dorothy A. Stewart has shown (Mem. and Proc. Manchester Lit. 

 and Phil. Soc, lviii., Session 1913-1914, No. 1) that in Ligia oceanica 

 the respiratory functions of the exopod, "if they exist, are very 

 slight," and that though the animals eventually die if constantly sub- 

 merged in water, either fresh or salt, they possess "a considerable 

 adaptability in regard to immersion in sea- water." 



fj.. H. Stoller. "On the organs of Eespiration of the Oniscidae," 

 Zoologica, Heft 25, 1899. 



^Chilton. Jour. Linn. Soc. London, Vol. 32, 1915, p.435. 



