THE SIPIIONAL APERTURES 



165 



near to the siphonal apertures of an Anodonta or Uoiio. In 

 many cases (e.g. Psammolna, Tellina, Mya, genera which burrow 

 deeply in sand) both the siphons are exceedingly long, sometimes 

 considerably longer than the whole shell. In some cases the two 

 tubes are free throughout their entire length, in others they be- 

 come fused together before their entrance within the shell (Fig. 

 74). In other genera, which do not burrow (e.g. Ostrea, Pecten, 

 Area, Mytilus), the siphons are rudimentary or altogether absent 

 (Fig. 75). 



The number and arrangement of the branchiae varies con- 



FiG. 74. — Solecurtus strigillatus L., Fig. 75. — Mytilus edulis L., attached by its byssiis 



Naples: s.af, afl'ereiit siphon; (By) to a piece of wood : F, foot ; S, aual siphon, 



s.ef, efferent siphon, the two the brancliial siphon being below it and not 



uniting in SS e.xternally to the closed. (After Mcibius.) 

 shell. X J. 



siderably. It appears proliable that the different degrees of 

 complication of the gill indicate degrees of specialisation in the 

 different groups of Pelecypoda, in other words, assuming that a 

 simpler form of gill precedes, in point of development, a more 

 complicated form, the nature of the gill may be taken as indicat- 

 ing different degrees of removal from the primitive form of 

 bivalve. 



