542 Comparative Animal Physiology 



circulation of essential fluids. Burrowing worms, particularly sipunculoids 

 and some annelids, depend on proper changes in the coelomic fluid pressure 

 and turgidity of the body. In Arenicola, for example,*^ pressure in the 

 anterior body cavity when the animal was at rest averaged 12.2 cm. H2O, 

 when active, 36 cm. H2O, and when burrowing, 27 cm. HoO; removal of 

 some coelomic fluid delayed burrowing. In Glycera 220 pressure in the 

 coelomic cavity of the animal at rest was 0.5-2 cm. H2O and on activity it 

 was 8 cm. H2O, whereas in Neanthes the pressure rose from 1-2 cm., resting, 

 to 15 cm. H2O on activity. In Neanthes, the pressure in the dorsal vessel 

 tends to be higher than it is in the coelom, and it tends to increase more 

 with activity. 



In sipunculoids high turgidity is essential for burrowing; eversion of the 

 proboscis is accompanied by elevated internal pressure, but the retractor 

 muscles can withdraw the proboscis against a high pressure. In the body 

 cavity of large specimens of Sipunculus, pressures as high as 600 cm. H2O 

 have been recorded, and in small Phascolosoma pressures ranged from 2.8 

 cm. of body fluid in the relaxed state to 108 cm. when contracted.--" Sipun- 

 culus is unable to burrow if the posterior part of the body wall is paralyzed 

 by deganglionation, but if a ligature is then placed ahead of the paralyzed 

 posterior half, the anterior portion can disappear rapidly into the sand.^^^ The 

 magnitude and speed of change of pressure are probably greater in sipun- 

 culoids than in any other animal group. 



Hydraulic mechanisms are also important among echinoderms. In holo- 

 thurians, for example, there are several fluid compartments, the water-vascular 

 or ambulacral system, the sinus or pseudo-hemal system, the lacunar or blood 

 system, the coelomic cavity, and the respiratory trees. Cilia maintain some 

 fluid flow in the smaller passages; the filling and emptying of the respiratory 

 trees has little effect on pressure in the coelomic fluid. The anterior ambu- 

 lacral system consists of two portions separated by a valve: the tentacular 

 ducts, which feed fluid into the tentacles; and the Polian complex of radial 

 ducts, ambulacral ring, and Polian vesicles, which is a fluid reservoir for the 

 tentacular system. Pressure can vary independently in these two portions 

 of the system: in Thyone pressure is normally higher in the tentacular duct 

 (10-25 cm. H2O) than in the Polian complex (3-5.5 cm. H2O in radial 

 duct).^^" In the body cavity of a relaxed Thyone pressure is low (0-2.5 

 cm. H2O), but on muscular activity pressure may rise to as much as 37 cm. 

 H2O, and ch^ges occur independently in the Polian vesicle and body 

 cavity. ^^" 



PHYSIOLOGY OF HEARTS 



Types of Hearts. Any system for circulating a mass of fluid requires a 

 repeating pump. To assure that fluid goes in a constant direction the pump 

 either must be equipped with suitable valves to prevent backward flow or 

 must compress its contained fluid in a continuous progressing wave. Some 

 hearts employ one of these propulsion mechanisms, some use both. Morpho- 

 logically hearts may be classified as (1) chambered heart, (2) tubular heart, 

 (3) pulsating vessels, or (4) ampullar accessory heart.'*'* 



Chambered Hearts. Chambered hearts occur mostly among vertebrates 

 and in some molluscs. Vertebrate hearts are equipped with valves and sue- 



