Plan of Ascidian. 235 



Figure 3. 



Plan of Ascidian ; from Bronn's ' Klassen und Ordnungen des Thierreichs,' pi. xvii., 

 fig. 9, after Allman, * Fresh-water Polyzoa,' p. 44, fig. 6. 



Arrows mark the directions taken by the inhaled and exhaled 

 currents, and a figure of a heart has been placed in a line with the 

 ventral margin of the branchial sac, with which one end of the 

 vasiform heart is very directly connected by means of a vessel in 

 relation with the structure known as the ' endostyle/ and indicated 

 in this figure by the thick vertical line passing down the left side 

 of the branchial net- work. Functionally, the direction of the heart's 

 action, and of the currents of blood it propels, are alternately reversed 

 in the Ascidians, as shown by Milne Edwards (Memoire sur les 

 Ascidies Composees, Mem. de FInstitut, tom. xviii., 1842, p. 228); 

 but homologically, this vasiform prolongation along the line of the 

 endostyle is probably to be considered as representing the veins 

 which in the Lamellibranchiata bring blood back to the heart from 

 various parts of the mantle, as well as from the gills (see p. 60, supra). 

 The concavity of the curve described by the first segment of the 

 intestine, together with the stomach, looks in most Ascidians 

 towards the line of the endostyle, and away from the single nerve 

 ganglion seen in this figure between the inhalant and exhalant ori- 

 fices. This disposition however of the parts is not constant in the 

 Ascidians, not being, for example, maintained in Ascidia affinis, the 

 species described under Preparation 32, p. 66 siq^ra. 



If this figure be so viewed as that the nerve ganglion shall occupy 

 the same relative place as the branchial or parieto-si^lanchnic gan- 

 glion I, of the Lamellibranchiate figured on Plate V, and the arrow 

 at the inhalant orifice occupy the same position as the line a , and 

 the arrow at the exhalant the same position as the line c in that 

 figm*e, the theory here adopted as to the homologies of the several 

 structures in the two classes of animals under comparison will be 

 made plain. The single nerve ganglion of the Ascidian will then 

 be seen to hold much the same relations to its exhalant and inhalant 

 tubes as the parieto-splanchnic ganglia of the Lamellibranchiate do 

 to the anal and branchial compartments of its mantle cavity; the 

 branchial sac of the Ascidian will correspond with the inferior or 



