152 



KNOWLEDGE 



[August 1, 1891. 



Anothei- point of interest remains to be noted in regard to 

 the position of the floral parts. The ovary is twisted in 

 such a way as to cause the truly anterior portions of the 

 flower to hold a posterior position and rhr n-rxu, but in the 

 foregoing description, and in Fig. IV., no account has been 

 taken of this. The labellum, for instance, has been spoken 

 of as the anterior petal and two anterior stamens of the 

 outer whorl united, whereas the adjective posterior would 

 be the correct one to use. 



SWIMMING ANIMALS. 



By E. Lydekker, B.A., Cantab. 



IN our last article we discussed the various structural 

 modifications by means of which the members of 

 difierent groups of animals are enabled to fly, or, in 

 other words, to swim in the aerial ocean. From the 

 observations recorded there, it is evident that all the 

 creatures adapted for this peculiar mode of life have been 

 specially modified for that purpose ; flight thus always 

 being a power-which has been specially acquired, and not 

 one which was an original attribute of any group of 

 animals. 



It is our purpose in the present essay to notice in a 

 somewhat similar manner the various adaptations of the 

 structure of certain animals whereby they are enabled to 

 swim in the denser medium of water. And here we shall 

 find that w^hile there is conclusive evidence to show that 

 in many instances this power is an acquired one, yet there 

 are others which lead to the belief that in certain groups 

 it is a primitive fimction. Some clue as to the groups in 

 which this power of swimming is an acquired one, and 

 those in which it is a primitive one, is afforded by the 

 different modes in which aquatic animals breathe. Thus 

 in fishes the air necessary to oxygenate the blood is 

 obtained from that dissolved in the water itself by its 

 constant passage over those pecuUar comb-like organs, 

 highly charged with blood, known as giUs ; such animals 

 having, therefore, no occasion to come to the surface of 

 the water to breathe. In other animals, however, such as 



Fig. 1. — The Common- Gkampis, ok Kii.i.eb Whale. 



Whales and Grampus (Fig. 1), atmospheric air is breathed 

 directly by means of lungs, necessitating visits at longer 

 or shorter intervals to the surface, and it is in such 

 instances that we may safely infer that the adaptation 

 to an aquatic life has been gradually developed from 

 ancestors whose normal habits were terrestrial, since 

 otherwise the gills would never have been lost. That 

 animals whose original mode of life was a purely aquatic 

 one have tended in some cases to assume a terrestrial 

 existence is proved by the case of the Common Frog, 

 which commences life as a giU-breathing, swimming 

 creature, which is to all intents and purposes a fish, and 

 ends by being an air-breathing reptile, as much at home 

 on land as in the water, although retaining the power of 

 swimming. On the other hand, the Seals and Otters show 

 us how an originally terrestrial type of animal has become 

 adapted to pass a large part of its time in the water, which 

 has become its natural element. 



The tei-m " Swimming Animals " is, of course, a very 

 wide one, since a considerable proportion of animals whose 

 nonnal habits are teiTestrial can, on occasion, swim with 

 more or less facility. Our appUcation of the term will, 

 however, in the main be restricted to those creatures 

 which pass a considerable amount, or the whole of their 

 time in the water, and which have accordingly been more 

 or less specially modified for that kind of life. Again, in 

 many groups of purely aquatic animals, it is sometimes 

 difficult to say which are ti-ue swimmers ; a certain 

 I number leading an active life when young, and becoming 

 more or less complete tixtiu'es in adult life. We shall 

 ' commence our survey with the Invertebrate Animals, 

 treating them, however, in a somewhat briefer manner 

 than the Vertebrates, and alluding only to some of the more 

 striking adaptations of certain parts of the body for the 

 piurpose of swimming. 



All are familiar vdih those disc-like masses of pellucid 

 gelatinous matter so often thrown up on our sea-beaches, 

 and popularly known as .JeUy-Fishes ; but to see them in 

 their full beauty we should look down from the bowsof a large 

 vessel traversing the warmer oceans. There they may be seen 

 in countless multitudes, extending as far down in the water 



as the eye can 

 penetrate, and by 

 dayhght present- 

 ing various tints 

 .^> of pink and pur- 



- .-.--. pie, while by night 



they are often 

 p ho s p bore s cent. 

 These Medusas, as 

 they are techni- 

 cally called, are ge- 

 latinous creatures 

 shaped somewhat 

 like an umbrella, 

 the " handle " 

 being formed by a 

 mass of thick ten- 

 tacles hanging 

 down in the water. 

 They swim by the 

 alternate contrac- 

 tion and expan- 

 sion of the " um- 

 breUa" or beU, the 

 diameter of wftiich 

 may considerably 

 exceed a foot. 

 Medusse 



