108 Zi OVP BYR HS: 
highest developement in the Echinoderms. Among zoophytes, the 
Hydra forms the first step upward, in which the digestive cavity is a 
mere sac, which will work equally well turned inside out, and the 
mode of reproduction is extremely simple. From this group, we 
pass to the Actinia, in which there is a distinct stomach and a series 
of fleshy lamellw around the internal cavity,—the first rudiments of 
an isolation of the functions of digestion and generation,—but the 
circulating fluid is only the elaborated chyle, mingled with more or 
less water from without. A step farther, and we find the develope- 
ment of separate organs for the functions of the liver and of genera- 
tion in the Echinoderms, and in some species a circulating system. 
Whether the Acalephe or Zoophyta are highest in the scale, we do 
not intend to indicate above. The young of some zoophytes are 
acalephs in form, even to organs of sight, and the young of some 
acalephs are much like polyps: moreover, the adult acaleph is often 
quite analogous in its radiately subdividing alimentary channels, to 
the same in the Fungia. 
Through the Bryozoa, the infusoria are connected with the 'Tuni- 
cata and the other Molluscs; and through the Rotifera and Entozoa, 
they connect with the Articulata, thus passing each way, out of the 
true Radiate type, into that which characterizes the higher sub- 
kingdoms. The Bryozoa, Rotifera, and Entozoa, may be arranged 
in the sub-kingdom Radiata, or with the Mollusca and Articulata, 
whose types of structure they exhibit, though under a Radiate form. 
The Echinoderms, although so strikingly peculiar in some species, 
—the Echini,—yet, through the Holothuriew, bear closely upon the 
Articulata; while the Acalephs incline toward the Mollusca. 
The animal kingdom is throughout a network of affiliations, yet 
there are main trunks and larger branches, to which the smaller 
anastomosing ramifications are subordinate. Systems of 3s and 5s, 
look pretty to the mathematical eye, and have some foundation in 
nature ; yet, in tracing affinities, it is better first actually to ascertain 
Crustacea, the same or analogous organs, together with the mouth, become lateral, owing 
to the developement and projection of the cephalic ganglion—and its accompaniments 
constituting the head—beyond to one side of the circular series of the natatory plates ; 
these natatory plates, about the Rotifer’s mouth, becoming, at the same time, the branchie 
and the attached maxillary organs about the Crustacean’s mouth. The transition from 
the Crustacean to the Radiate type is also shown in the passage of the Caligi into the 
Epizoa. This subject admits of a long series of illustrations, which are reserved for 
another place. 
