TUNICATA. 
43 
Tunicata. Many of the former live in colonies, and are commonly 
mistaken for sea-weeds, although they are really animals of high 
organisation. The Brachiopoda have two shells like bivalve 
Molluscs, only the valves are dorsal and ventral, instead of 
right and left. One valve is frequently perforated, hence the 
name of "lamp-shells." Tunicates, or Ascidians, are worthy of 
Fig. 14.— Commox AVhelk, a typical Gastropod, 
a, siphon ; foot ; c, tentacles ; r7, eyes ; e. operculum. 
Fig. 15. — Left Valve of Giant Clam, a typical Bivavle. 
(Length, 36 inches ; weight, 154 lb. ; weight of the two valves, 310 lb.) 
the best attention of the visitor, on account of their evident 
affinity with the vertebrate stock, of which they may be 
regarded as a degenerate type. It is in the larvie (fig. 17) that 
the evidence of vertebrate relationship is most conspicuous. 
A fine series of very large shells and some interesting 
Cephalopods in spirit occupy four upright cases at the south end 
