496 LECTURE XX. 



oblong depressed cone, attached by its apex to the ventricle, the 

 apex being perforated by the auriculo-ventricular aperture, and 

 adherent by its base to the auricular sinus, which might be said to 

 conduct into the common peritoneal cavity, since the delicate tunics 

 of the visceral sinuses appear to take the place of a peritoneum. 

 The auricular w^alls are disposed in small plaits or folds, radiating 

 from the apex : and the longitudinal or radiating folds are puckered 

 by transverse folds. The figures ii and iii, pi. 3, of CCCIII., which are 

 magnified to the same degree, exemplify two of the diff'erent states 

 in which the plicated auricles are found in different individuals, and 

 so far exemplify the extent of dilatation and contraction of which 

 these complex cavities are susceptible ; but they have presented 

 more extreme differences of size in other specimens, and tliey must 

 possess considerable powers of altering their capacity. It is, there- 

 fore, probable, that, when the circulating fluid is accumulated in 

 unusual quantity in the pallial and visceral sinuses, the longitudinal 

 auricular fibres evert and expand the margins of the basal aperture 

 to which the delicate tunic of the sinuses is attached, that the fluid is 

 drawn by a kind of vermicular movement or peristaltic suction into 

 the auricles, and thence propelled by successive contraction of tlio 

 circular fibres into the ventricles. Fi'om the ventricles the blood 

 may be driven through the ramifications of the pallial and visceral 

 vessels into the more or less irregular and capacious sinuses, and so 

 returns slowly back to the heart. The auricles are relatively smaller 

 in Lingula than in Terebratula. 



The part of the vascular system which most conspicuously presents 

 the usual ramified form, is that which is on the inner layer of the 

 mantle. In Terebratula and Orbicula the branches are given off 

 from four trunks on one pallial lobe, and two trunks on the other lobe : 

 they terminate at the periphery in a cicumpallial vessel. In Liiigula, 

 there are two trunks on each lobe, which are longer and their branches 

 are more regular and parallel, and some of them terminate in blind 

 ends. The primary external parallel branches in Lingula Andebardd 

 give off a series of short loops or caeca.* In all a more slender body f 

 accompanies the larger vessels throughout their ramifications : in all 

 the generative cells (testes or ovaria)are developed within the pallial 

 vessels and apparently from the more slender ramified body.| 



Wherever the blood is exposed in its delicate vessels and sinuses 

 to the sea-water, a respiratory action must go on ; most actively at 

 the part where the blood is most subdivided, viz., at the ciliated 



* CCCI. pi. 23. fig. 16. 



t CCCI. pi. 22. fig. 11. pi. 23. fig. 11. CCCVIII. tab ii, fig. 14 b. 



X CCCI.pl. 22. fig. 16. 



