THE PELECYPODA. 117 



The Pelecypoda, or LameUibranchiata are 

 characterised by the absence of a head and 

 tooth - band (Odontophore), and by the 

 possession of a shell composed of two pieces, 

 or valves. The Pelecypoda are classified 

 according to Gill-structure. The Gills, or 

 breathing organs, are composed of numerous 

 filaments arranged parallelwise, and pendant 

 from an axis, like the teeth of a comb. 

 Collectively, they form plates, or lamellae, 

 running lengthwise of the body, there being 

 two of such plates, or Gills, upon either side. 

 In the lowest forms of the Pelecypoda, the 

 filaments are straight, but in the higher types, 

 after descending, are bent laterally upwards 

 and nearly parallel ; in many groups their 

 ends become united laterally, whilst in others, 

 they fuse with the adjacent parts of the body. 

 The Gill, being thus bent, forms two nearly 

 parallel surfaces, which are kept relatively 

 apart by "junctions," either in the form of 

 bars at suitable intervals, or by vertical plates, 

 or Septa. In like manner, the filaments, in 

 relation to one another, are either free, or 

 are retained in perfect parallel and open order 



