440 



GENERAL CONSIDERATIONS 



the contractile vacuole. Respiration takes place through the general 

 body surface in all metazoan forms up to and including the annelids, 

 except in the brachiopods and bryozoans. In the case of the last two 

 groups, as also in many of the annelids, respiration occurs through the 

 surface of certain projecting tentacles. In the echinoderms a form of 

 skin gill is developed, as described in Chap. XXXV. In mollusks gills 

 are developed which are outfoldings of the surface of the body and into 

 which is projected a network of blood vessels. In a few mollusks, 

 however, gills are absent, and respiration takes place through the wall 

 of a hollow sac, or lung, and also through the surface of the mantle. In 

 crustaceans external gills are found; in the insects, a system of tracheal 

 tubes; and in the arachnids, book lungs, or book gills. In all of these 



Fig. 293. — Diagrams showing types of glands. A, simple acinous gland. B, simple 

 tubular gland. C, simple tubular gland showing coiling. D, compound acinous gland. 

 E, compound tubular gland. 



various types the blood is a medium of transport for the gases within the 

 body, except in insects, where the circulatory system is poorly developed 

 and where the gases are distributed by the complicated tracheal system. 



In the lower chordates respiration takes place through the walls of 

 pharyngeal clefts; and in the lower vertebrates, up to and including part 

 of the amphibians, this is still the case, gills of different types being 

 developed on the walls of those clefts. In the adults of most amphibians, 

 however, it has been seen that the gills are replaced by lungs, while in 

 higher vertebrates lungs are the only functional respiratory organs, 

 reaching in the highest forms the greatest complexity and the largest 

 amount of surface exposed (Fig. 294). 



476. Circulatory System. — In the protozoans circulation takes place 

 within the cell; in the lower Metazoa, up to and including the Platy- 

 helminthes, from cell to cell. In the sponges it is aided by ameboid 



