34 THE POPULAR SCIENCE MONTHLY. 



the blood, and is so thin and delicate, and so richly supplied with 

 blood-vessels, that, v?hen moistened, it readily admits of the interchange 

 of gases. During the breeding-season the abdomen of the female frog 

 is so distended by eggs that there is no room for the inflation of the 

 lungs, and respiration is entirely carried on through the skin. 



In those marine and aquatic animals of small size whose bodies are 

 not covered by impervious shells, respiration may take place, as in 

 plants, over the whole surface of the body ; and in many groups, the 

 larger and protected members of which are furnished with highly- 

 specialized respiratory organs, these organs may be entirely wanting in 

 the smaller naked forms. This is the case, for instance, with many of 

 the naked moUusks. 



In place of a closed system of vessels for the circulation of the 

 blood, this fluid may, in parts of its course, find its way through the 

 spaces among the viscera, and in the Tunicata a pulsating heart keeps 

 in motion a haemal fluid, which is nowhere confined by distinct blood- 

 vessels, but fills and circulates through all the intra-visceral spaces of 

 the body. In most mollusks, again, the musculur movements of the 

 various parts of the soft body aid the heart in maintaining the circu- 

 lation ; and in the Polyzoa both vessels and heart are lacking, and the 

 movements of the blood are due to muscular contractions, aided in 

 many cases by cilia. 



The salivary glands are frequently wanting in marine animals ; the 

 water swallowed with the food taking the place of saliva. The liver 

 and other appendages to the digestive tract are wanting in many of the 

 lower animals, and a parasitic life may entirely do away with the need 

 for them, even in an animal which belongs to a highly-specialized group. 

 The ordinary gasteropod mollusks have digestive organs which are 

 almost as highly specialized as those of a vertebrate ; but " entocon- 

 cha," a strange parasitic gasteropod, has no jaws or teeth, no salivary 

 glands or liver, no anus, or any other specialized appendage to the 

 digestive cavity, which is a simple pouch, which is not divided into 

 oesophagus, stomach, and intestine, and the animal lives inside the body 

 cavity of a holothurian, attached to the wall of its stomach, and is' 

 nourished by the fluids which it sucks from the digestive cavity of its 

 host. In the sponges and hydroids there is no digestive cavity distinct 

 from the body cavity, and the food is received directly into the latter. 

 Even the mouth is wanting in many parasitic worms, and the liquid 

 food is absorbed through the outer surface of the body, precisely as in 

 plants, and some parasites put forth root -like processes which penetrate 

 the tissues of the host and absorb its juices. 



The nervous system and organs of special sense are wantijig in 

 many of the lower animals, and in the fresh-water hydra the reproduc- 

 tive organs are not formed in specialized internal glands at definite 

 points, but resemble those of plants in making their appearance at 

 many points upon the surface of the body. 



