46 COMPARATIVE ANATOMY. 



the radial cseca (Siebold). The anus, in Echinodermata, is 

 placed alongside of mouth in Pentacrinus (sea- lily); upon 

 dorsum, either directly opposite mouth in Toxopneustes, or to 

 one side, as in Spatangus ; at posterior extremity, as in Holo- 

 thuria (sea-cucumber) ; or it may be absent, as in Astropecten. 



Hepatic secretion formed from gastric follicles. 



MOLLUSC A. Polyzoa. The mouth is surrounded by prom- 

 inent ciliated, protrusile tentacles. The canal, after leaving 

 a muscular stomach (gizzard), turns once upon itself to open 

 by an anus near the mouth. 



Hepatic secretion formed from gastric cells. 



Brachiopoda. The two respiratory arms (q. v.) are placed 

 one on either side of the mouth. The canal may be simple, 

 as in Lingula, or with stomach, as in Terebratula. Anus lat- 

 eral, hidden between lobes of mantle; absent in Waldheimia. 



Tunicata. The oral opening at base of respiratory cham- 

 ber (q. v.). From this point the canal, after commonly ex- 

 panding into a stomach, curves at first downward, then 

 upward, to terminate within the cavity of the mantle, an 

 excretory chamber, common to both alimentary and genital 

 apparatus, conducting thence the ejecta. 



Lamellibranchiata. Mouth placed near anterior adductor 

 muscle, with tentacular flap on either side. Stomach large, 

 and lined with papillae. Intestine often convoluted and inti- 

 mately associated with other viscera. The rectum commonly 

 traverses the heart (Ostrea, oyster, an exception); anus upon 

 dorsum near hinge line, and behind position of posterior 

 adductor muscle. Liver a large diffuse organ made up of 

 distinct acini. 



Gasteropoda. Mouth simple, protrusile, and tooth -bearing. 

 Gullet muscular. Stomach present, simple, as in Helix (snail) ; 

 gizzard-like, as in Limnceus; compound, as in Aplysia (sea- 

 hare) ; occasionally armed with teeth (q. v.). Intestine con- 

 voluted, terminating upon right side, near respiratory orifice. 

 Salivary glands present, placed either around oesophagus or 

 stomach. Hepatic secretion always present; rarely secreted 

 from intestinal cells or follicles, but as a rule from well- 

 developed, lobulated liver, emptying by excretory ducts into 



