62 
tacles. The two plates may now be compared to the covers 
of a book attached to one another along the back, but 
having the remainder of their edges free. They are com- 
posed of elongated prismatic cells. While the arms of the 
lophophore project from between their distal free edges, the 
fundus of the stomach (c) with the attached funiculus (d) may 
be seen projecting from between the proximal edges. The 
funiculus is still short and thick. 
As development proceeds the lophophore (fig. 8, b ) becomes 
greatly increased in size, and the minute tubercles of the pre- 
vious stage now show themselves as unmistakable tentacles. 
The whole of the body, however, with the exception of the pro- 
jecting arms of the lophophore, and the cul-de-sac of the 
stomach with the funiculus, is still enclosed between the two 
valve-like plates which have hitherto kept pace with the 
general enlargment of the bud. 
In the further progress of the development the plates 
cease to keep pace with the growth of the other parts, and 
by the time the polypide has attained its full size and adult 
form they cover but a small portion of the whole, and may 
now be seen as the peculiar shield-like organ (fig. 2, b ) which 
performs so important a part in the morphology of Rhabdo- 
pleura. 
In thus following the development of Rhabdopeeura it 
is impossible not to be struck with the resemblance of the 
valve-like fleshy plates to the mantle of a lamellibrancliiate 
mollusc, whose lobes lie, as here, to the right and left of the 
body, instead of being placed dorsally and ventrally, as in 
the Brachiopoda. The conviction is thus forced upon us that 
the right and left plates of the young Rhabdopeeura may, 
after all, be the representatives of the mantle-lobes of a 
Lamellibranch. 
If this be so, a new light is thereby thrown upon the 
morphology of the Polyzoa, whose relations must then be 
admitted to be more intimate with the Lamellibranchiata than 
with the Brachiopoda, with which the Polyzoa have of late 
years been associated. Indeed, the most important differ- 
ence between a Polyzoon and a Lamellibranch will be found in 
the direction of the intestine, which, instead of finally bending 1 
towards the closed or dorsal edge of the mantle, retains its 
original neural flexure, and runs at once towards the open or 
ventral edge. The absence of a heart and specialised respi- 
ratory apparatus is of comparatively slight importance, and, 
like the degraded condition of the nervous system, only shows 
the low stage of development on which the Polyzoa rest. 
1 See Huxley, article “Mollusca,” in ‘English Cyclopaedia,’ and “Lectures 
on Comparative Anatomy.” 
