475 
good reasons to believe that the Porifera or sponges (Parazoa) 
had a different origin from the Metazoa, the former from the 
Choanoflagellata and the latter from the Ciliata. Thus, it fol- 
lows that multicellular animal organisms, with their attendant 
sexual method of reproduction, and the formation of ova and 
spermatozoa, has arisen at least twice independently of one an- 
other. The production of multicellular organisms was the most 
momentous step in animal evolution, and the fact that it 
occurred more than once in the animal world is of supreme 
significance. 
In each great phylum illustrations of homoplasmy, or con- 
vergent or parallel development, can be found. The simplicity 
of the organization of Protozoa is not favorable to the recogni- 
tion of such development, but students of those organisms could 
cite good examples. 
Stephenson’s * paper on Indian Oligochaeta is of great 
value from our standpoint, as he shows how parallel and con- 
vergent development has gone on in a number of genera, and 
he gives convincing evidence for polyphyly in some of them. I 
have contended for this in certain genera of Homoptera. This 
important paper should be consulted by all interested in this 
subject. 
Many cases of homoplasmy could be cited from the Mol- 
lusca, but I will confine myself to those cases in which branchiae 
or gills are replaced by a lung or lung cavity. The animals in- 
cluded in the order Pulmonata all have a pulmonary sac. They 
are placed in two sub-orders, and are considered by many as 
having two distinct origins—the Basommatophora, originating 
from the Opisthobranchiata, sub-order Tectibranchiata, and the 
Stylommatophora from the sub-order Nudibranchiata. Most of 
the Prosobranchiata possess branchiae, but some possess pulmo- 
nary sacs; of the latter, some families (Cyclophoridae, Cyclosto- 
matidae, Aciculidae) are placed in the sub-order Monotocardia, 
while others Helicinidae, Hydrocenidae, Proserpinidae are 
placed in the sub-order Diotocardia. Here we have four groups 
in which the branchiae have been replaced by lung cavities all 
having independent origin. Even if we disagree with the above 
* Pro. Zoo. Soe. Lond. 1921, pp. 103-142. 
