KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. NIO 3. 65 



freely into the mantle cavity; 2) coalesce either with each other or 3) with the 

 siphonal fold in addition. When they häng freely the gills repeat the conditions of 

 a primitive ctenidium such as occurs in its simplest shape in the Protobranchia and 

 in a more advanced stage in Arcidae, Trigoniidae and most of the P seudolamelli- 

 branchia. In higher forms freely hanging gills are met with in Astarte and Cardita 

 (Rice 1898), and I have found this condition prevailing even in Gallista, Rupellaria 

 and Cyrena. 



I have found a similar individual variation of the gill fixation only in Septifer 

 (fam. Mytilidae). In different specimens of a single species e. g. S. excisus, S. bilo- 

 cularis, the gills are either completely free or have their tips attached to the mantle. 



As to the upper edges of the reflected lamellae these may be, in the Chamidae, 

 wholly or partly free from the body or attached to it throughout their length. 

 Different combi nations of the mode of attachment, even in the same species, are 

 exemplified in the descriptive part of the present inquiry. 



Both Anthony and Grieser have observed the peculiar partial fixation and 

 disconnection of the margins of the gill, but the fact that an individual variation 

 exists in these respects has not been pointed out by them. The construction that 

 Anthony puts upon the phenomenon seems to be somewhat nnsatisfactory, as he 

 considers it to be due to a »tendance qu'ont les branchies de ces animaux a rompre 

 leurs connexions avec les parties avoisinantes, a devenir compétement libres et flotter 

 dans la cavité palléale ...» He states three cases: 1) »les solutions antero-internes 

 existent seules, 2) les solutions internes et externes existent seules, 3) les branchies 

 flottent librement dans la cavité palléale». 



Contrary to Anthony I consider the freeness of the gills as a primitive 

 condition and their fixation as a secondary one, because the former case is confined 

 to primitive forms of Lamellibranchia, because it precedes the definite stage in the 

 ontogenetical development and lastly because a reduction from an accomplished stage 

 must be considered more improbable than the retention of a primitive stage. 



I also remember of the individual variation mentioned above in the fixation 

 of the gills. To this individual variation in Chamidae (as well as in Septifer) we 

 find a parallel — at least as far as the coalescence of the ends of the gills is con- 

 cerned — in the specific variation in Callista and Mytilus or in the generic variation 

 in the Veneridae family as a whole. In other words: the gills of Chamidae (and 

 Septifer) are, as far as the points mentioned above are concerned, still undefined 

 and subject to individual variation, while in other groups they have developed to 

 specific or generic constancy of the same characteristics. In Chamidae, so to speak, 

 a first step to the consolidation of the respiratory organs in the direction mentioned 

 is taken, though as yet it is a mere tentative groping and therefore does not result 

 in any definite organization. 



In dealing with the gills of Chamidae we have also to consider, as an im- 

 portant point, their intimate relation to the blood system. This has not been dealt 

 with in literature, and it may be said as a general verdict that little is known about 



K. Sv. Vet. Akad. Handl. Band 59. N:o 3. 9 



