236 PHYSIOLOGY OF THE INVERTEBRATA. 



tlie lower VcrUiljrata. In tliese animals the structure of 

 special interest is the dilated pharynx (see Fig. 17). A 

 constant stream of water enters the mouth and passes into 

 the pharynx, whose walls are richly supplied with blood- 

 vessels, and it is through the walls of the pharynx that 

 absorj^tion takes jilace. The tentacula serve as an accessory 

 organ of respiration. 



In the Polyzoci the protrusible parts of the body also 

 absorb oxygen from the surrounding water. MacMunn has 

 examined Lcpralia foliacca and finds that it contains an 

 abundance of chlorophyll mixed with a lipochrome. The 

 chlorophyll is also accompanied by a second pigment, 

 probably chlorofucin. Flustra foliacca also contains a chloro- 

 phylloid pigment. 



The Brachiopoda. 



In these animals the blood is contained in branched 

 sinuses in the perivisceral cavity. In the sinuses of the 

 ciliated tentacle-bearing arms and of the inner wall of the 

 mantle, the blood is purified by being brought into close 

 osmotic relation with the water in which these animals live. 



Very little is known concerning the presence of respiratory 

 pigments in the Brachiopoda. 



The Mollusca. 



The aquatic Mollusca have well-developed branchiaa usually 

 enclosed within branchial chambers. These branchias are broad 

 and plate-like ; and the water in contact with them is changed 

 by means of vibrating cilia. In the Brancliiofjastcropoda the 

 branchia3 are dendritic, instead of plain and plate-like. 



The Ccphcdopocla are divided into two distinct orders — 

 the Dibranchiata with two gills, and the Tctrcd)ra7ichiaia 

 with four gills in the mantle cavity. The water is conducted 

 to the branchial chambers of the Dihranchicda (which is the 

 highest of the two -orders) by means of the infundibulum 

 whose opening is situated beneath the head. 



