252 Introduction to A^iimal Alorfhology. 



walls, an outer homogeneous, an inner muscular.* It 

 receives an anterior stem from over the oesophagus, 

 and sends off lateral branches ; the vein collects the 

 blood of the lacunae and of the mesenteric spaces. 

 The two lateral vessels are united for a short distance ; 

 each divides into two branches, anterior and posterior ; 

 the former going to the dorsal lobe of the mantle, and 

 giving off medial and lateral branches ; the dorsal branch 

 supplies the reproductive glands, the other traverses 

 the ilioparietal septum, over the excretory organ, and 

 supplies the ventral lobe of the mantle. In the wall 

 of the mantle are two accessory hearts or pyriform 

 sacs receiving and transmitting blood ; these are pro- 

 bably homologous to the venous accessory organs in 

 Cuttlefishes. There is no pericardial segment of the 

 body cavity, unless the space included in the ilio- 

 parietal septum be considered as such. 



One or two pair of excretory organs exist, which 

 are modified segmental organs. In Rhynchonella 

 one pair is dorsal and one ventral. The dorsals are 

 absent in Lingula and Terebratula ; in the former the 

 ventral organs lie between the two layers of the ilio- 

 parietal septum. They open by a tubular part ex- 

 ternally near the bases of the arms, and by a radially 

 folded funnel, \vhich has been mistaken for a heart, 

 into the body cavity. In Terebratula the tubular part 

 is short, and the mouth separated by the occlusores ; 

 the tubular part is lined by glandular epithelium. 

 This tube is the usual channel for the extrusion of the 

 sexual products. 



The nervous system consists of a large hypo- 



* Lingula has no heart {Semper), and the circulation is carried on by 

 cilia in the large vessels. 



