6o 



OUTLINES OF NATURAL HISTORY. 



can be recognised by the presence of tlie mouth (o) with 

 its membranous feelers. The mouth is destitute of teeth, 



Fig. 25.— Diagram of the anatomy of Mya. o Sleuth ; g Stomach ; i In- 

 testine, surrounded hy the' liver ; v Vent ; h Gills ; h Heart ; s s 

 Breathing-tubes ("siphons") ; /Foot; n Nervous system ; «rf Front 

 adductor muscle ; jxZ Hinder adductor muscle. 



and leads through a gullet into a stomach ((/). From the 

 stomach proceeds a long winding intestine (/) which is 

 surrounded by a well-developed liver, and which finally 

 terminates in a distinct vent. The vent is placed at the 

 hinder extremity of the body, and is so situated as regards 

 the breathing-tubes (siphons) that all undigested particles 

 of food are carried away by the outgoing current of water 

 which has passed over the gills. The nervous system (n) 

 consists of three little nervous masses, connected by cords. 

 There is a distinct heart (h), which drives the pure blood, 

 which has come from the gills, to all parts of the body. 

 The gills are in the form of membranous plates (h), two 

 on each side of the body, and having their surfaces 

 covered with minute hair-like processes, which lash to and 

 fro in constant vibrations, and sweep the water over the 

 gills. 



We are thus led to consider how the water reaches the 

 gills, for the animal would die of suffocation unless it 

 could constantly get a supply of fresh water. In order 

 to understand this, it is necessary to know how the Mi/a 



