THE SIPHONAL APERTURES 



165 



water, near to the siphonal apertures of an Anodonta or Unio. 

 In many cases (e.g. PsammoMa, 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 become fused together before their entrance within 

 the shell (Fig. 74). In other genera, Avhich do not burrow 

 (e.g. Ostrea^ Fecten^ Area, Mytilus), the siphons are rudimen- 

 tary or altogether absent (Fig. 75). 



The number and arrangement of the branchiae varies con- 



s^f... 



Fig, l\. — Solecurtus stripillatus 

 L., Naples: s.af, afferent si- 

 phon; s.ef, efferent siphon, the 

 two uniting in SS externally to 

 the shell, x i 



Fig. 75. — Mytilus edulis L., attached by its byssus 

 (By) to a piece of wood : F, foot; S, anal siphon, 

 the branchial siphon being below it and not 

 closed. (After Mobius.) 



siderably. It appears probable 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. 



