ANATOMY AND PHYSIOLOGY. 61 



that proceed from the heart were dilated into auricles, the 

 rudiments of the Lamellibranchiate heart would also be 

 established. This idea of an arrest of a bilateral growth 

 is somewhat strengthened by Krohn's description, already 

 quoted, of the development of Ascidia mamillata, in which 

 the young at first has two distinct lateral atrial spaces and 

 two lateral excurrent orifices ; the spaces ultimately coalesce, 

 as do also the orifices, the tendency to bilateral development 

 terminating at a very early period. 



If this view of the homologies of these organs be correct, 

 then the cloacal, or that which has been uniformly designated 

 throughout this communication the ventral surface, will cor- 

 respond to the dorsal region of the Lamellibranch ; and con- 

 sequently the opposite margin will be the ventral aspect, and 

 the so-called right and left sides will have to interchange 

 appellations. Thus the excurrent tube will become dorsal, 

 and the iiicurrent ventral, as they are in the Lamellibran- 

 chiata, and, without any great disturbance of the parts, all 

 the viscera will assume their proper positions. 



Before the probability of this determination of the homo- 

 logical relations can be admitted, it is necessary to ascertain the 

 true nature of the ganglion, which, as we have seen, is placed 

 between the respiratory tubes. In the Polyzoa the ganglion 

 is placed on the rectal aspect of the oesophagus, immediately 

 below the mouth, and gives its nerves to the tentacles and to 

 the oesophagus in the direction of the mouth, but none to the 

 " endocyst " (mantle) or to any other organ. Therefore it can 

 scarcely be homologous with the ganglion in the Tunicata, 

 which distributes all its nerves to the walls of the respiratory 

 tubes (which are mere prolongations of the mantle) and to 

 the mantle itself. In the Lamellibranchs, however, there is a 

 ganglion (or a pair of ganglions), namely the branchial, the 

 most constant in these animals, situated upon the posterior 

 adductor muscle, which, besides supplying the gills, gives 

 nerves to the dorsal portions of the mantle and to the respira- 

 tory tubes, parts which are the undoubted homologues of 

 those which receive the nerves from the ganglion in the 

 Tunicata. It therefore seems impossible to avoid the con- 

 clusion that the ganglion in the latter is the true representa- 

 tive of the branchial ganglion in the Lamellibranchiata : 

 ganglia supplying homologous parts must likewise be homo- 

 logous. 



This determination of the nature of the ganglion agrees 

 well with its position, which in relation to the respiratory 

 tubes is almost precisely similar to that of the branchial 



