92 



EVERSS COMPARATIVE ANATOMY. 



3. A distinct heart is first seen in the Crustacea. 



4. In the gasteropodous mollusca, as the snail, the rectum passes 

 through the heart. 



5. There is no portal circulation in the avertebrata. 



6. The heart of a fish consists of a single auricle and ventricle, 

 and the blood of the porta is distributed to the liver and kidneys. 



7. All the amphibia have at first the single heart of a fish, but 

 the caducibranchiate species terminate life with the double heart of 

 a reptile. 



8. The heart of birds consists of four distinct compartments, as 

 in mammalia, but rather more perfect, owing to the existence of its 

 fleshy valves. 



9. The heart of the higher warm-blooded mammalia, even that 

 of man, in the course of its development, represents the several 

 grades which constitute the permanent types of the lower animals. 



CHAPTER XV. 



RESPIRATORY SYSTEM IN THE INVERTEBRATA. 



The respiratory organs are sometimes placed in the interior of 

 the body in the form of lungs sometimes towards its exterior in the 

 form of lamellated, ramified, pectinated, tufted, ciliated, or pin- 

 nated processes, termed gills, or branchiae, and a third form of re- 

 spiratory apparatus is obtained by the development of a system of 

 tracheal tubes, ramified to an exquisite degree of minuteness, and 

 widely spread through all the organs of the body. Animals pro- 

 vided with lungs generally breathe atmospheric air, whilst in those 

 furnished with branchiae respiration is accomplished by means of 

 water. An exception to the former, however, is met with in the holo- 

 thuria; and to the latter, in the terrestial Crustacea. It will appear 

 in the progress of this article that some of the vertebrated animals 

 commence life by a branchial respiratory and terminate it by a pul- 

 monary, and that others enjoy a mixed form of respiratory organ all 

 through. In fact, nature seems almost to have exhausted her 

 ingenuity in the construction and development of respiratory 

 apparatus. 



Cyclo-neura. — In the lowest classes of this division the only 

 respiratory organs detectible are small cilia pervading the entire 

 surface, but so minute as to require high magnifying powers to 

 render them visible. The tentacula and the whole surface of the 

 body are subservient to the function of respiration in the polypifera. 

 In the asterias and sea-urchin among the echinodermata, the water 

 passes into and out of the cavity lined by peritoneum, in which the 

 viscera are lodged. In the holothuria the water is alternately re- 



