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POPULAR SCIENCE REVIEW. 
crescentic form is not at all obvious ; but, according to Professor 
Allman, tbis species really presents no departure from this 
type, though the arms of the crescent are obsolete. The 
digestive tube consists of mouth, oesophagus, cardiac and 
pyloric cavities of stomach, intestines, pylorus, and anus. The 
muscular system is well developed ; Allman describes eight 
different kinds, the most obvious of which are (1) the powerful 
retractor muscles by means of which the entire Polypide is 
drawn within the cell ; (2) the rotatory muscles of the crown, 
whose action it is to rotate the crown and depress the lobes ; 
(3) the anterior parieto-vaginal muscles, by means of which the 
invaginated* endocyst is dilated, and kept permanently inverted; 
and (4) the posterior parieto-vaginal muscles, whose action 
“ steadies the sheath, and regulates its position during the pro- 
trusion of the Polypide.” 
The Fresh-water Polyzoa increase by true ova, by gemma?, 
and by statoblasts. The ova, which are formed in a true ovary 
after escaping into the perigastric cavity, are there most pro- 
bably impregnated by the free spermatozoa ; but how the ciliated 
embryo escapes from the cells it is not easy to determine, seeing 
that no orifice has yet been detected through which it might 
effect an exit. The gemmae (or buds) originate in the endocyst 
as small tubercles which project into the perigastric space, but 
soon appear as little vesicles projecting from the exterior of the 
parent cell ; in these vesicles the Polypides are gradually deve- 
loped. The points at which the vesicles sprout forth, determine, 
of course, the ultimate form assumed by full-grown specimens. 
The statoblasts are bodies of a very peculiar kind (see Plate XIV., 
figs, b, c, cl, e ), which are produced as small buds upon the pos- 
terior funiculus, i.e., a curious cord-like body attached to the base 
of the pyloric cavity of the stomach at one end, and to the 
testis at the other. At first these statoblasts are colourless 
and almost transparent; in time they assume a dark brown 
colour, and acquire a leathery consistency, when they separate 
from the funiculus and fall free into the perigastric space, 
where it is probable they remain until they can gain an exit 
when the Polypide dies, and the destruction of its soft parts 
allows an escape through the orifice. This, I believe, is the 
only way of accounting for the escape of the statoblasts in 
most of the genera, as Professor Allman is convinced ; but in 
the case of Cristatella the statoblasts remain clustered to- 
gether by their marginal spines, and are not emitted singly ; 
on the destruction of the tissues of the colony they are 
liberated, and either float on the surface of the water or attach 
themselves to submerged weeds. 
* i.e., sheathed, or drawn backwards into itself, as may be done with the 
finger of a glove, ■ 
