THE LAMEIiLTlSl!ANCIIIATA. 





The last common and distinctive peculiarity oi* the Ascidians 

 which I have to mention, is one which acquires importance only 

 from its constancy. The middle of the haemal wall of the 

 pharynx, from near the oral to the oesophageal end, in all these 

 animals, is pushed out into a longitudinal fold, the bottom of 

 which projects into a blood sinus, and has a much thickened 

 epithelial lining. Viewed from one side, the bottom of the fold 

 consequently appears like a hollow rod, and has been termed 

 the " endostyle ' : (m, Fig. 11). The functions of this structure 

 are unknown, but it has been noticed in all genera of Ascidians 

 hitherto examined. 



In the next group, that of the Lamellibkanchiata, which 

 comprises those creatures which we know as mussels, cockles, 

 and scollops, and all the fabricators of what are commonly 

 known as bivalve shells (except the Brachiopoda), w T e meet with 



Fiff. 12. 



Fig. 12. — Sectional diagram of a freshwater mussel (Anodon). A A, mantle, the right 

 lobe of which is cut away ; B, foot ; C, branchial chamber of the mantle cavity ; D, 

 anal chamber; /, anterior adductor muscle; II, posterior adductor muscle; 

 a, mouth; 6, stomach; c, intestine, the turns of which are supposed to be seen 

 through the side walls of the foot ; d, rectum ; e, anus; /, ventricle ; fj, auricle ; h, 

 gills, except i, right external gill, largely cut away and turned back; k, labial palpi : 

 /, cerebral ; m, pedal ; n, parieto-splanchnic ganglia ; o, aperture of the organ of 

 Eojanus ; p, pericardium. 



an important advance in organization. In all these animals, the 

 body, as is exemplified by the diagram (Fig. 12) of a freshwater 



D 



