54 COMPARATIVE ANATOMY. 



brownish, as in Acalephse; yellowish, greenish, or red, as in 

 Annelida ; or blue, as in Crustacea. The hue is generally 

 resident within the corpuscles; but occasionally, as in certain 

 Annelida, is contained within the serum. 



Vascular currents are maintained by ciliary or muscular 

 action. The latter may be secured everywhere in the con- 

 tractile walls of the vessels, as in Annelida, or located in 

 specialized portions (pulsatile vesicles, or 'hearts'), as in the 

 majority of the higher forms. 



Systems of blood-vessels may be complete when the arte- 

 ries and veins unite through an intermediate network of 

 small vessels (capillaries), as in most Annelida and Verte- 

 brata; or incomplete, when, in place of the capillary vessels, 

 spaces (lacunae), irregularly limited by muscles and viscera 

 intervene, as in Mollusca. 



(2) Where no circulation is present, the interchange of 

 gases takes place in all parts of the animal (aeration). But 

 where one exists, a separate apparatus is devoted to the reno- 

 vating function, which is then called respiration. Aeration, 

 however, may occur in conjunction with respiration, as in 

 intestine of Cobitis (loach), and dorsal integument of Rana 

 (frog).* 



The respiratory apparatus is subject to great variation in 

 form and position. The plans of construction are dependent 

 upon the type to which the animal conforms, and the nature 

 of the medium in which it lives. This last may be (1) Nutri- 

 tive Juices: examples include embryos of viviparous animals, 

 as in Paludina (fresh-water snail), Anableps, Blennius, Selachia, 

 and Mammalia. (2) Water containing air: examples include 

 Pisces, young Batrachia, and all oviparous Invertebrata, ex- 

 cluding pulmonate Gasteropoda, and certain Articulata, as 

 pulmonate Arachnida, and some larval forms of Insecta. 

 (3) Air : examples include eggs of Reptilia and Aves, adult 

 Batrachia, Reptilia, Aves and Mammalia, and those excepted 

 in the preceding enumeration. 



The aerating medium may be taken to the blood, entering 



* For excretory fuuctiou, see p. 64. 



