232 MARINE ANIMALS 



tractile bodies of salpas and pyrosomas are more or less tube-shaped. 

 This form of locomotion is highly developed by the squids, which 

 outswim and catch fishes on which they feed. 



Oar-shaped jointed limbs are a frequent form of swimming organ. 

 Among pelagic annelids the parapodia are employed for this purpose 

 (Alciopidae, Tomopteris, Fig. 53c). In the pelagic crustaceans, a 

 varying number of limbs are used; in small forms like the copepods 

 (Fig. 59) and small larvae, the second pair of antennae fulfills this 

 function and may be much enlarged ; among schizopods and numerous 

 pelagic amphipods, numerous limbs may be employed for swimming. 

 The pelagic decapod crustaceans are distinguished from their benthic 

 relatives by the broadening of two (Sergestes) or four {Polybius) 

 pairs of posterior thoracic limbs. 11 Among the pteropods two parts of 

 the foot are transformed into oars. Sea turtles and penguins row with 

 their forelimbs. 



Undulation of the body is the most common mode of swimming. 

 Pelagic cephalopods and the rays swim by the undulation of lateral 

 fins, to which, in the former, is added the darting produced by sudden 

 expulsion of water by the muscular mantle. In the heteropod snails 

 a median ventral fin is undulated. In most fishes, whales, and seals the 

 whole body or the posterior part of the body undulates. The resistance 

 to the waves produced by these motions drives the fish forward, and 

 the component acting on the lower surface neutralizes the force of 

 gravity. Thus the body form of the large pelagic sharks, the bony 

 fishes without air bladders, the porpoises, and seals, represents a spe- 

 cial type, almost round with a slight ventral flattening, reached by 

 convergence in these several groups. 



Active motion is variously combined with other means of suspen- 

 sion. Swimming, as distinguished from motions which merely produce 

 suspension, may be defined as motion sufficiently active to render the 

 animal independent of the oceanic currents. Such swimming is never 

 produced by cilia or quivering motion, but is always dependent on 

 muscular exertion. This increases proportionately with the size of the 

 animal, and is accordingly greater, as a rule, in larger animals. Small 

 animals less than 1 cm. long (and hence all microscopic animals) are 

 incapable of such swimming. 



The effectiveness of muscular motion is such that other arrange- 

 ments for suspension are rarely combined with it. The large pelagic 

 rays (Mobula and Manta), reaching a breadth of 7 m., with their 

 depressed bodies, perhaps form an exception. For the rest, among 

 small free-swimming animals the suspensory processes are placed in 

 the plane of motion (for example, in Calocalanus, Fig. 53g) ; thus they 



