154 



BREATHING ORGANS IN AMPIIINEURA 



found (Hily in the Ami^hinenra , while one or rarely two (never 

 more) pairs of" liranchiae, occur, with various important modi- 

 tications, in the vast majority of the Mollusca. 



Aiivpltineura. — In Chiton tlie branchiae are external, forming 

 a long row of short plumes, placed synnnetrically along each side 

 of the foot. The numl)er of plumes, at the base of each of whicli 

 lies an osphradial patch, varies from about 70 to as few as 

 G or 7. A\nien the plumes are few, they are confined to the 

 pL)steri(jr end, and thus approximate to the form and position of 

 the la-anchiae in the other Amphineura. In Chaetoderm<(, the 

 branchiae consist of two small feather - shaped bodies, placed 

 synnnetrically on either side of the anus, which opens into a sort 

 of cloaca within which the l)ranchiae are situated. In JSfcomenia 

 the branchiae are still further degraded, consisting; of a single 



lT'"{ 



Fid. 61. — Terminal portions of the Amphineura, illustrating the grailiial degrailation 

 of the branchiae, and their grouping round the anus in that class. A, Chiton 

 (Uemtarthrum) setulosus Carp., Torres Str. ; B, Chiton {Lejjtochiton) benthus Hadd., 

 Torres Str.; C, Chaetodi'nnn : D, Xeomenia ; a, anus; ///•, br, branchiae; k, k, 

 kidneys ; p, pericardium. (A and B after Haddon, C and D after Hubrecht.) 



liUMch of tilaiuents lying within the cloaca, while in F'roncomcni(( 

 there is no more than a few irregular folds on the cloaca-wall 

 (Fig. Gl;. 



In the Frosohranchiata, symmetrically paired l)ranchiae occui 

 only in the Fissurellidae, Haliotidae, and I'leurotomariidae, in the 

 former of which two perfectly equal branchiae are situated on 

 either side of the l)ack of the neck. These three families taken 

 together lV)rm the group known as ZygohrancJiiata} In all other 

 families the asynnnetry of the bod}' has prolmbly caused one of 

 the branchiae, the right (originally left), to become aljorted, and 

 consequently there is only one branchia, the left, in the vast 

 majority of marine Prosobranchiata, which ha-s^e been accordingl}' 

 ^ iv-yov, a yoke, from the symmetrical position of the branchiae. 



