346 MB. E. SPEUCE ON INSECT-MTGEATIOKS IN SOUTH AMERICA. 



tral, as tliey are in the Lamellibrancliiata, and, without any 

 great disturbance of the parts, all the viscera will assume their 

 proper positions. 



Before the probability of this determination of the homological 

 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 " (man- 

 tle) or to any other organ. Therefore it can scarcely be homo- 

 logous with the ganglion in the Tunicata, which distributes all its 

 nerves to the walls of the respiratory tubes (which are mere pro- 

 longations of the mantle) and to the mantle itself In the La- 

 mellibranchs, 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 

 respiratory 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 conclusion that the 

 ganglion in the latter is the true representative of the branchial 

 ganglion in the Lamellibranchiata : ganglia supplying homologous 

 parts must likewise be homologous. 



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 ganglion. And 

 we thus find in the nervous element a corroboration of the above 

 suggestion as to the homological relation of the branchial sac. 



Notes on some Insect- and other Migrations observed in Equa- 

 torial America. By Richard SpRcrcE, Esq. Communicated by 



the President. 



[Read June 6, 1867.] 



In endeavouring to trace the distribution of plants in the Amazon 

 valley, and to connect it with that of animals, I have been struck 

 with the fact that there are certain grand features of the vegeta- 

 tion, which prevail throughout Cisandme America, within the tro- 

 pics, and even beyond the southern tropic, — features independent 

 of the actual distribution of the running waters, partly also of the 



