154 



BREATHING ORGANS IN AMPHINEURA 



CHAP. 



only in the Amphineiira^ while one or rarely two (never more) 

 pairs of branchiae, occur, with various important modifications, 

 in the vast majority of the Mollusca. 



Amphineiira. — In Chiton the branchiae are external, forming 

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

 of the foot. The number of plumes, at the base of each of which 

 lies an osphradial patch, varies from about 70 to as few as 6 or 

 7. When the plumes are few, they are confined to the pos- 

 terior end, and thus approximate to the form and position of 

 the branchiae in the other Amphineura. In Chaetoderma^ the 

 branchiae consist of two small feather-shaped bodies, placed 

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

 of cloaca within which the branchiae are situated. In Neomenia 

 the branchiae are still further degraded, consisting of a single 



ir-: 



Fig. 61. — Terminal portions of tlie Amphineura, illustrating the gradual degradation 

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

 {He miar thrum) setulosvs Carp., Torres Str. ; B, Chit07i (Leptochiton) benthus 

 Hadd., Torres Str.; C, Chaetoderma ; D, Xeomenia ; a, anus; br, hr, branchiae; 

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



bunch of filaments lying within the cloaca, while in Proneomenia 

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

 (Fig. 61). 



In the Prosohranchiata^ symmetrically paired branchiae 

 occur only in the Fissurellidae, Haliotidae, and Pleurotoma- 

 riidae, in the former of which two perfectly equal branchiae 

 are situated on either side of the back of the neck. These 

 three families taken together form the group known as Zygo- 

 hranchiata} In all other families the asymmetry of the body 

 has probably caused one of tlie branchiae, the right (originally 

 left), to become aborted, and consequently there is only one 

 branchia, the left, in the vast majority of marine Prosobran- 



' ^1^701', a yoke, from the symmetrical position of the branchiae. 



