MOLLUSCOIDA TUNICATA. 115 



161. The nervous system consists of a single principal 

 ganglion, which, in those groups that have the valves 

 united by a hinge, is connected with a chain of ganglia 

 passing round the gullet. 



162. Development. The sexes are sometimes distinct; 

 in other cases they appear to be united in the same in- 

 dividual. The young are believed to be able to swim 

 about by the action of their fringed arms. 



163. Divisions. They are divided into two groups 

 the Articulata and Inarticulate, In the Articulata the 

 valves are united along the hinge line by teeth and 

 sockets. The intestine ends in a blind sac. The mantle 

 lobes are united dorsally. The Terebratula belongs to this 

 division. In the Inarticulata the valves are not united 

 by teeth. The intestine is provided with an anal opening. 

 The mantle lobes are not connected. The Lingula is an 

 example of this group. 



CLASS 2.Tunicata, or Ascidioida. 



164. The Tunicata (Lat. tunica, a cloak) are so called 

 because they are covered with a leathery integument. 

 They are also called Ascidioida (Gr. askos, a wine bag), 

 because many of them are shaped like a double-necked 

 bottle. When touched, they discharge a jet of water; on 

 this account they have received the popular name of sea- 

 squirts. They are all marine animals, and may generally 

 be found on our coasts attached to sea-weeds, or shells, 

 especially after a storm. They are easily known by their 

 two prominent apertures. 



Some of the Ascidians are single, or simple. These 

 are fixed to some solid object, such as a submerged stone. 

 The social Ascidians form a colony by budding. They 

 are connected by a tube, through which the blood common 

 to the colony flows. In the compound Ascidians the 

 tests are joined together into a mass, but there is no 

 common circulation. Other groups float about on the 



