PROCEEDINGS OF SOCIETIES. 
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organs out of all danger, on the slightest appearance of pain or discomfort; an 
active locomotive system, in the higher tribes, and usually complicated organs of 
defence were alluded to, as means of avoiding risk of irreparable bodily injury. 
Instances were taken from lower orders, showing that even among undoubted 
animals these characteristics gradually decline, the cone-like glands attached in 
numbers by slender pedicles to the back of Doto among the mollusca, combining in 
themselves the functions of both liver and lung, easily broken off, and, if lost, easily 
and rapidly renewed ; the reproducible limbs of Crustacea, and the sensation of 
this tribe, blunted by their hard tegumentary covering, and, in every case, imperfect 
through the diffusion of the nervous centres—acute sensation being rendered less 
necessary, on account of the smaller value to life of the reproducible limbs, whose 
integrity it is intended to preserve. Dr. Thomson then described, as bearing on 
the same point, the Hydroid Zoophyte, a central tubular common column, beset 
with multitudes of distinct flask-shaped bodies, formed of a homogeneous gelatinous 
granular substance, each hollowed out into a simple digestive cavity, the aperture 
surrounded by- a whorl of arms for the capture of food—these polyps or stomachs, 
all exactly similar, all connected with the tree-like central column and canal; and 
each contributing digested nutriment for its support and extension, without any 
appearance of localized organs, every part of the general gelatinous mass performing 
the functions of nutrition, of respiration, and of reproduction. The structure of 
this being was adduced as one of the most perfect instances of the vegetative repeti¬ 
tion of similar parts , met with in the animal kingdom. With the complete 
localization of organs, and their combination in an unity of purpose in the higher 
animals, was contrasted the luxuriant vegetative repetition in the higher plants— 
each leaf an epitome of all the parts of the entire tree, complete in itself, and 
perfect in the performance of all vegetative functions; what in animals was an 
indication of imperfect development, becomes in plants a sign of high perfection. 
Could we imagine an oak rooted in the ground, and exposed to all the winds and 
fires of heaven, with an essential heart localized in a single branch? could we 
imagine an elm deprived of the power of reproducing the foliage stripped from it 
by the gales of autumn? 
At the public meeting on 7th March, a second lecture was given on the same 
subject. Dr. Thomson described more fully the various tribes of Zoophytes and 
their allies. First, the hydroids and sertularians, with a gelatinous body, usually 
enclosed in a free homogeneous horny tube, unconnected in any way with the 
animal, whose heads are freely protruded from the perfectly free margins of the 
polype cells. The ova contained in many cases in capsules, formed, as in plants, 
of compressed branches—but each polype capable of transformation into an ovum— 
the ovum assuming, in some forms, the character of a free medusoid. He objected 
to the usual definition of the term “ovum” (“Cyclopaedia of Anatomy,” &c.), and 
contended that it might rather be defined as a mass of cells separated from the 
parent, and containing a cell capable of being influenced, as to its multiplication, 
by a heterologous cell. In this sense he considered the medusoids true ova, and 
advanced, in proof, instances in which arrest in development of the ovum, and 
formation of the embryo within the capsule, render the ovoid characters more 
distinct. He compared the formation of these ova in the egg capsule of the genus 
campanularia to the formation of ovules round the free central placenta in the 
primulacese, and described capsules analogous to anthers, with their contained 
pollen, in the same genus. The hydroids are usually rooted to the ground, gently 
waving in the water currents. Connected with the hydroids, he referred to the 
acalephse, an order closely related to them, though apparently differing so widely. 
The internal development of this series does not correspond with the prestige of 
their external appearance, the large variegated disk is simply a gelatinous loco¬ 
motive apparatus, increased in size, in order to overcome, by the large quantity of 
fresh water contained in it, the extra weight of the important parts of the animal, 
and to reduce it to a density equal to that of sea water. The only anomalous 
elevation in its structure being the presence of a very simple nervous system, and a 
simply developed ear, the disk is provided with a regular muscular arrangement. 
The important parts of the medusse surround the mouth. In some of the simpler 
forms they resemble closely a single hydroid polype, attached to a locomotive 
