496 
REVIEWS. 
that the “ Hydroids, are localized within narrow boundaries, with as 
much precision as the higher orders of the class.” The Diphyidce , 
he thinks, furnish only apparent exceptions to this rule. But for the 
present he considers it premature to generalise on the subject. 
Part V., on the Homologies oe the Badiata, ends at once the 
volume and the Monograph. It includes only six pages, and is 
divided into two short sections, of which the first discusses general 
homologies, while the second enters on the special homologies of the 
classes. 
In the first section, after some cautions, as to the position in 
which, when studying their homologies, these animals should he 
placed, Professor Agassiz re-asserts his former conclusion that the 
body of every Badiate is essentially a sphere, “loaded in every 
direction with those structural differentiations which determine the 
peculiarities of organic structures.” This sphere resolves itself into 
a number of wedges, or ‘ spheromeres,’ “ arranged symmetrically 
around a vertical axis.” These wedges form, superficially, a number of 
alternate zones, reaching from pole to pole. “ The so-called mouth 
is always placed at one of these poles, and from it radiate the most 
prominent organs, in consequence of which I have called this side of 
the body the oral, or aetinal area, and the opposite side the aboral, 
or ahactinal area while the zones, ambulacral and interambulacral, 
“ differ chiefly in the differentiation of the substance, and the position 
of different systems of organs alternating with one another at the 
periphery of the body.” 
In the second section the author endeavours to show that a radi¬ 
ate arrangement of the parts, such as that just described, may be 
traced in the classes of Acalephs, Polyps, and Echinoderms, respec¬ 
tively. The longitudinal ambulacral tubes of the last-mentioned 
class, the body-chambers of the Polypes, and the radiating canals of 
the Medusae, (or, at least, some of them), are here stated to be 
strictly homologous with one another. The holes by which the 
chambers of the Polypes communicate, not far below the oral disk, 
are “ homologous to the marginal circular tube of the Acalephs, and 
are actually to be considered as short tubes through narrow walls, 
leading into wide radiating chambers.” Lastly, the gastric sac of 
the true Polypes is homologous to the oral apparatus of the Jelly¬ 
fishes, and is compared to the “ neck of a bottle, which in Polyps 
would be turned inside, while in Acalephs it is turned out and 
divided into a number of distinct lobes.” 
Intimately connected with the subject-matter of this portion of the 
work is the more detailed survey taken in Part III. of the “ Homo- 
logical Belations of Aurelia and Echinoderms.” Our readers are 
already aware how Professor Agassiz resolves the radiating canals of 
Aurelia into two systems, one ‘ambulacral ’ and one ‘interambulacral,’ 
the latter being in direct connection with the genital pouches. The 
elements of these two systems alternate with one another, respec- 
