2 3 8 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



fish, but different in the two cases. The squid is pro- 

 vided with a horizontal, arrowhead-shaped caudal fin 

 (not shown in the diagram), which is not flapped from 



Fig. 153. — Diagram showing geDeral structure of a squid : mc, the mantle 

 cavity ; s, the siphon ; g, gills ; z'0, ink bag ; isk, internal shell ; e, eye ; 

 oe, oesophagus ; k, kidney. The arrows show the direction of currents. 



side to side, as in fish, nor up and down, in the man- 

 ner of whales, but locomotion is effected by waves propa- 

 gated backward or forward — in the one case giving 



rise to gentle forward, in 

 the other to gentle back- 

 ward motion. These grace- 

 ful movements may be 

 ^-71_ watched in an aquarium. 

 The short-bodied cut- 

 tlefish, on the contrary, 

 uses its long, flexible, mus- 

 cular arms for crawling on 



Fig. 154.— Diagram of a medusa :n », the bottom, or even for 

 nerve ; »z, mouth ; st, stomach \rt. .. . 



radiating tubes. climbing up rocks. 



CCELENTERATA. 



Medusas. — As we are taking only the most strik- 

 ingly different modes, we pass over the echinoderms 

 and take next the acalephs or medusa. The transparent 



