Introduction to Ajiimal Morphotogy. 25 1 



central) are the homologucs of the ventral adjuster of Wald- 

 heimia. 



The mouth is a transverse slit between the arms ; 

 beneath it is a prominence or lower lip, the homologue 

 of the foot of Otocardians. The short, undifferentiated 

 pharynx and oesophagus pass in front of the crural 

 processes of the shell between the occlusores, and end 

 in a stomach, which may be oval, tubular, fusiform 

 (Crania), or lenticular (Lingula) ; from it, towards the 

 ventral valve, passes the intestine, which is either 

 short, with one neural flexure and with a caecal end 

 (Terebratulidae, Rhynchonellidae), or long, coiled, tra- 

 versing the liver, and ending in a papillary anus 

 (Lingula, Discina, Crania). The first form is a second- 

 ary degradation, since in Thecidium, an impervious 

 cord from the caecal intestine passes to where the anus 

 would be. The intestine is fixed by a longitudinal 

 mesentery of two parts, dorsal and ventral, extending 

 from behind the oesophagus to the end of the attach- 

 ment of the occlusor muscle. Two transverse parti- 

 tions, the remains of the inter-metameric septa, also 

 suspend the intestine. The first, gastro-parietal, con- 

 sists of a middle (absent in Lingula) and two lateral 

 portions ; the second, or ilio-parietal, is farther back. 

 The liver is a multilobar mass made of many csecal 

 pouches, with many ducts in Crania, four in Lingula, 

 two on each side opening into the stomach ; in Lingula 

 two lobes are in front, and two behind the gastro- 

 parietal band. 



The circulation is lacunary, the vessels (excepting 

 a few primary ones) having no proper walls. The 

 heart is a saccular organ, lying on the stomach be- 

 hind the middle gastro-parietal bands; it has two 



