XVIll] 



SHAPE JN EELATION TO HABITS 



391 



at both ends ; ovs, sharp) refers to the shape of the body, which 

 is long, flattened and pointed at both ends (Fig. 185). It is remark- 



FIG. 185. Amphioxus lanceolatus from the left side, about twice natural size. 

 After Lankester. The gonadic pouches are seen by transparency through 

 the body- wall ; the atrium is expanded so that its floor projects below the 

 metapleural fold ; the fin-rays of the ventral fin are indicated between the 

 atrial pore and anus. The dark spot at the base of the fifty-second 

 myotome represents the anus. 



able that we here meet for the 

 first time with a shape very 

 common among Vertebrates, 

 but very uncommon elsewhere 

 in the animal kingdom, viz. a 

 laterally compressed form with 

 narrow ventral and dorsal 

 regions and deep sides. It 

 is common to find animals 

 with broad backs and bellies 

 and narrow sides, but rare 

 except amongst Vertebrates to 

 find the reverse condition. In 

 consequence of this peculiar 

 shape AmpMoxus falls on its 

 side when it ceases moving. 

 It burrows in the sand, lying 

 with its mouth just protrud- 

 ing, and as its lips are fringed 

 with ciliated rods (Fig. 186) 

 a current is produced which 

 brings new water to the gills 

 and with it small swimming 

 organisms which serve as food. 

 At night Amphioxus often 

 leaves its burrow and swims 

 about, returning instantly to 

 the sand if alarmed. It can 

 burrow with either the head 

 or tail. 





A. Velum of Amphioxus seen from the 

 inside of the pharynx. After Lan- 

 kester. 



v.sp, sphincter muscle of velum. 

 v.t. velar tentacles lying across the 

 oral opening. 



B. Oral cartilages of Amphioxus. After 

 J. Miiller. The basal pieces lie end 

 to end in the margin of the oral 

 hood, and each basal piece sends up 

 an axial process into the correspond- 

 ing oral cirrus. 



