METAZOA MOLLUSCA. 201 



The gills have become more complex, consisting of 

 many longitudinal tubes connected by cross tubes. 



The mouth is provided with two pairs of palpi and 

 leads into the so called "body" containing the stomach, 

 liver, most of the intestines, and the reproductive organs. 

 The long, cartilaginous rod called the crystalline style 

 may give rigidity to this part of the animal. The intestine 

 leaves the body, passes dorsally under the beak and 

 through the heart. It terminates a short distance from 

 the upper tube of the siphon lying in the path of the out- 

 going current of water. 



The more specialized members of the group are Pholas, 

 Aspergillum, and Teredo. 



Pholas dactylus (Nos. 377, 378) has a large opening in 

 the anterior end filled by the foot. This genus has the 

 habit of boring. The valves are united by an external 

 ligament, and the hinge has two plates to strengthen the 

 union but no teeth. 



Aspergillum when young has a bivalve shell. As the 

 animal grows older the siphon grows to a large size, and 

 is covered by a limy tube in which the tiny reduced 

 bivalve shell becomes imbedded, as seen in No. 379, 

 specimen at the right. At the end of the tube is a sieve 

 surrounded by a frill. At the other end are one or more 

 frills which are broken off in the specimen. In Asper- 

 gillum the mantle is bag-like, having the two si phonal 

 openings and one at the anterior end. 



Teredo has a long body ; at the larger end is a little 

 bivalve shell which is without teeth or ligament. The 

 mantle is drawn out into a long siphon near the end of 

 which are organs probably used for boring into wood. It 

 is a strange freak that causes the animal to live in wood, 

 since it never uses it for food. Early in life, however, it 

 begins to bore, and lines its tunnel with a calcareous 

 secretion as seen in No. 380. It never leaves its tunnel 

 and depends for food upon the microscopic plants and 

 animals which are brought in the water. Many Teredos 



