536 



MUSCULAR SYSTEM. 



transparency and smooth surface, and under 

 the highest powers of the microscope present 

 neither an appearance of cross markings nor 

 of a linear arrangement of globules." Besides 

 the retractor muscles in the Bryozoa there like- 

 wise exists a muscular membrane which lines 

 the cell, and forms the parietes of the body, 

 in which fibres are distinctly apparent, running 

 transversely : these by their contraction com- 

 pressing the visceral cavity and the fluid which 

 it contains will tend to elongate the body of 

 the Bryozoon, and assist in effecting its protru- 

 sion ; although, as Dr. Farre supposes, this 

 process is principally accomplished by the 

 cooperation of the alimentary canal, which has 

 the power of straightening itself from the sig- 

 moid flexure, into which it is thrown when the 

 animal is retracted. 



The condition of the nervous system in the 

 Bryozoa has not been as yet made out, a cir- 

 cumstance at which no one will be surprised 

 who considers the extreme difficulty of micro- 

 scopic researches concerning the structure of 

 animals so minute as these ; but from the close 

 affinity that there decidedly is between these 

 animals and the Rotifera, there can be little 

 doubt that a similar arrangement exists in 

 both. 



In the ROTIFERA or Wheel Animalcules, the 

 nervous system, according to Ehrenber?, con- 

 sists of several filaments communicating with 

 minute ganglia dispersed in different parts of 

 the body, although without any obvious arrange- 

 ment or symmetrical disposition, so that the 

 muscular apparatus in these beautiful animals 

 presents itself in a very perfect state of deve- 

 lopement. The ciliated organs around the 

 mouth, which are apparently the representatives 

 of the ciliated arms in the Bryozoa, are retracted 

 by a special set of muscles derived from the in- 

 terior of the membrane that lines the shell and 

 circumscribes the visceral cavity, and the antago- 

 nists to these are the delicate parietes of the 

 visceral cavity itself, which acting upon the fluid 

 therein contained, causes the extrusion of the 

 ciliated lobes, whenever the wheel-like organs 

 are required to be put in motion * 



But besides the muscular bands, that, in 

 the Rotifera, are appropriated to the protrusion 

 and retraction of the wheel-bearing organs, 

 others are connected with a peculiar prehensile 

 apparatus placed at the hinder extremity of the 

 body, and forming an instrument of very great 

 importance in the economy of these creatures. 

 It consists of a prehensile forceps, the blades 

 of which are worked by distinct muscles; and 

 by the assistance of this onjan the action of the 

 wheel-like cilia is at once changed from that of 

 a locomotive power into a means of procuring 

 and seizing food. If these forceps are not 



* It seems more than probable that the tranverse 

 muscular fibres that occur in the parie.al membrane 

 of the Rotifera have been mistaken by Ehrenberg 

 for vascular canals, described by tha. observer as 

 emanating from a dorsal vessel ; such at least is 

 the opinion of Dr. Arthur Farre in the memoir 

 above referred to, an opinion which quite coincides 

 with the result of our owu observations upon this 

 subject. 



employed, the apparently rotatory organs pro- 

 pel the little animal rapidly through the water; 

 but does it choose to take hold of some foreign 

 body by means of its forceps, and thus anchor 

 itself in a given spot, the use of the wheels is 

 entirely changed, their rotation merely pro- 

 ducing currents or rather a powerful whirlpool 

 in the water, which sucks from a distance every 

 thing within its influence, and thus brings food 

 into the mouth. 



The next class of animals, the EPIZOA, pre- 

 sent us witli a beautiful series of gradations of 

 development, clearly demonstrating the insepara- 

 ble relation that must exist between the nervous 

 and locomotive systems. The Epizoa seem, 

 indeed, to be the osculant group interposed 

 between the Intestinal worms and the Articu- 

 lated classes, and exhibit in a permanent con- 

 dition the progressively improving external 

 articulated limbs, which are only permitted to 

 attain their full development in higher races of 

 the animal creation. The Epizoa, like the 

 Coelelmintha, are parasitic in their habits, living, 

 however, upon the external surface, and not in 

 the interior of other animals. They are prin- 

 cipally found fixed to the eyes, the skin, the 

 gills, or even the inside of the mouth of fishes, 

 or to the branchial organs of various forms of 

 aquatic animals, from which they suck the 

 materials necessary for their support, and at the 

 same time are freely exposed to the influences 

 of the surrounding medium for the purpose of 

 respiration. In the humblest of these parasites 

 the structure of the body is scarcely superior 

 to that of many Coelelmintha, suckers and pre- 

 hensile organs placed in the vicinity of the 

 mouth being their only means of adhering to the 

 surface upon which they live; but in the Ler- 

 neans the first appearance of outward limbs 

 begins to be perceptible, not as yet recognisable 

 as leys or locomotive agents, but not the less 

 on that account the first rude sproutings of 

 members that are to be by degrees perfected in 

 more highly privileged genera. Some of these 

 Lerneans, indeed, present most grotesque 

 shapes, and almost exactly resemble the em- 

 bryos of Vertebrate animals at the period when 

 the first buddings of limbs begin to project 

 from the sides of the body. This resemblance, 

 indeed, is far more real than it would at first 

 appear, inasmuch as there is a parallelism to 

 be established between the permanent condi- 

 tion of the Lernean and the transitory state of 

 the embryo at the corresponding period of its 

 development that is strictly physiological. The 

 condition of the nervous system in them both 

 is precisely similar, exhibiting in both cases 

 the nematoneurose type; the same rudimen- 

 tary condition of the muscular system is con- 

 sequently equally met with in the embryo and 

 in the Lernean, but as the nervous system in 

 the former is rapidly advancing to a more 

 exalted state of development, so do the limbs 

 and the muscles appertaining to them improve 

 in the same ratio. 



In the higher genera of EPIZOA minute 

 ganglia exist in connexion with the nervous 

 filaments, and in such the limbs are of course 

 more exactly formed and begin to sketch out 



