PROCEEDINGS OF SOCIETIES. 87 



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 

 primulaceas, 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 

 acalephae, 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 medusae surround the mouth. In some of the simpler 

 forms they resemble closely a single hydroid polype, attached to a locomotive 



