STRUCTURE AND DEVELOPMENT OF LOXOSOMA. 307 
of the Polyzoa, based on an investigation of the metamor- 
phosis of Actinotrocha; he states that “the dorsal surface 
in Polyzoa is indicated as in Phoronis by the line between 
mouth and anus/ 5 and gives the following explanation of his 
views on the larvae : “ The larvae of Brachiopoda and Polyzoa I 
regard as modified from the Trochosphaere by the earlier 
attainment of the relation of the ventral surface which in 
Phoronis is only accomplished during the metamorphosis.” 
In Actinotrocha, the larva of Phoronis, the surfaces 
correspond to those of ordinary Trochospheres, but their 
relations become much distorted by the formation (by invagi- 
nation) of a large ventral sac between mouth and anus, 
subsequently everted as the “ foot,” into which the alimentary 
canal passes. The Trochosphere bend of the alimentary 
tract is thereby exactly reversed, the “ foot ” becoming the 
body in which the gut forms a (J -shaped tube with a dorsal 
flexure. Caldwell’s suggestion is that this dorsal flexure of 
the alimentary tract has become already acquired during the 
larval condition of Polyzoa and Brachiopoda. In the 
developmental history of Loxosoma, however, there is not 
the slightest trace of the existence of a flexure of the alimen- 
tary canal towards the dorsal surface. We have already 
compared the larva of Loxosoma with that of Dentalium, 
an admitted Trochosphere; we have found that in both 
forms the mesoblast develops as two lateral masses near the 
surface containing the mouth and the anus. Further, the 
comparison already instituted between the dorsal organ of 
Loxosoma and the brain of Dentalium, between the sub- 
oesophageal ganglion of the former and the pedal ganglia of 
the latter, seems to leave no doubt that the dorsal organ of 
Loxosoma represents its prseoral lobe, and as such a part of 
the dorsal and not of the ventral surface. As an additional 
indication of the surfaces, I must insist on the position of the 
apertures of the nephridia between mouth and anus, and in 
front of the (supposed) suboesophageal ganglion. On Cald- 
well’s hypothesis, we must regard the adult ganglion as the 
brain, and the curious result follows that the head-kidneys 
