MUSCULAR SYSTEM. 



535 



arrangement ; such a disposition providing for 

 the extension or shortening or lateral inflexions 

 of the worm, and enabling the animals so con- 

 structed to move about with facility in the cavi- 

 ties wherein they reside. 



Nevertheless, at this, its first appearance in 

 the animal series, muscular fibre has not as yet 

 attained to the perfection of structure that it 

 offers in the higher classes. The fibres are as 

 yet indistinct, soft, and gelatinous ; they ap- 

 pear to be deficient in fibrin ; neither do they, 

 when examined with the microscope, present 

 Ihe transverse stria that are so characteristic of 

 the muscular tissue in a more advanced condi- 

 tion. The little fascicles are, moreover, ex- 

 tremely short, and run but a little distance 

 before they disappear, and are succeeded by 

 others. Their whole appearance, in fact, is 

 that of muscle in a rudimentary condition, and 

 very accurately resembles the nascent muscular 

 tissue \\hen it first becomes apparent in the 

 embryo of the vertebrate animal. The nervous 

 system accompanying this condition of the 

 muscles is extremely simple ; a delicate ring 

 surrounds the commencement of the oesopha- 

 gus, by which, perhaps, the muscles of the 

 mouth are associated during the act of imbibing 

 nourishment, and prolonged from this ring are 

 two long and thread-like nerves, one running 

 along the dorsal and the other along the ventral 

 aspect of the body, and passing quite from one 

 extremity to the other, but without any percep- 

 tible ganglionic enlargements in their course.* 

 Having, therefore, no brains or central masses 

 to which perceptions could be referred, localized 

 organs of sense are likewise wanting in all the 

 CCELELMINTHA. The BRYOZOA which, while 

 naturalists were ignorant of their more compli- 

 cated organization, were, until a recent period, 

 confounded with the Polyps, are, from their 

 entire structure, very justly entitled to a much 

 higher position in the scale of animals, and un- 

 doubtedly belong to the Nematoneurose type. 

 These little beings inhabit transparent cells of 

 very elaborate and delicate construction, from 

 the mouth of which they protrude the anterior 

 portion of their bodies when in search of food. 

 Although from their general appearance the 

 Bryozoa might easily be, and in fact have been 

 until recently, erroneously regarded as Polyps, 

 the differences between the two classes are ex- 

 ceedingly striking and important. The Bryo- 

 zoa, instead of having the tentacula that sur- 

 round the oral aperture quite simple and fila- 

 mentary, as the Polyps have, are furnished 

 with ciliated tentacula, and from the rapid 

 ciliary movement which is incessantly going 

 on, while the arms are expanded, strong cur- 

 rents are formed in the surrounding water, 

 all of which impinging upon the oral orifice 

 bring to the mouth such nutritive materials as 

 are necessary for the support of the creature. 

 The digestive apparatus is no longer a closed 



* The dorsal nervous cord of the Ascaris, first 

 described by Cloquet, (Anatomie des vers intesti- 

 naux,) seems to have been overlooked by the learned 

 writer of the article " Animal Kingdom," who has 

 arranged the Entozoa as being diploneurose ani- 

 mals. 



sacculus, as in the Polyps, but, on the contrary, 

 presents a very elaborate structure, consisting 

 of a gizzard, stomach, and distinct intestinal 

 canal, terminated by an anal aperture, though 

 which the fseces are ejected ; and, more- 

 over, the whole of the digestive viscera float 

 loosely in a visceral cavity wherein they are 

 suspended. In addition to this, the mouth of 

 the cell occupied by the Bryozoon is defended 

 by a most delicate and complicated opercular 

 apparatus, requiring a very perfect set of mus- 

 cles to perform all the required movements 

 connected with the protrusion and retraction of 

 the body, so that there is abundant reason for 

 separating the animals in question from the 

 Acrite Zoophytes. 



As relates to their muscular system, rudi- 

 mentary in its development as it must still ne- 

 cessarily be, many circumstances of great inte- 

 rest have been brought to light by the patient 

 investigations of Dr. Arthur Farre, to whose 

 valuable memoir* we are indebted for the fol- 

 lowing particulars, which we give at some 

 length, more especially as they will serve not 

 only to elucidate this part of our subject, but to 

 correct several important errors that have been 

 promulgated relative to the ROTIFERA, an 

 important class of animals next to be noticed, 

 of very analogous structure. 



1 or the purpose of drawing the protruded 

 Bryozoon into its retreat two distinct sets of 

 muscles are provided, one set acting upon the 

 animal and the other upon the flexible opercu- 

 lum that closes the cell. The muscles for the 

 retraction of the animal are contained in the 

 visceral cavity, and consist of two bundles of 

 delicate thread-like cords, the one set arising 

 from the bottom of the cell to be inserted about 

 the base of the stomach, the other also arising 

 from the bottom of the cell and passing up free 

 by the sides of the pharynx to be inserted around 

 the line of junction between this organ and 

 the tentacula. 



The muscles provided for the retraction of 

 the operculum consist of six flattened bundles 

 of fibres, which act upon the flexible portion 

 of the cell and a delicate circle of sete placed 

 around its orifice. It is at once evident there- 

 fore that the muscular system in the Bryozoa is 

 capable of great precision of action, and the 

 fasciculiare most accurately adjusted. Neverthe- 

 less if the intimate structure of this form of 

 muscle be investigated, it is found not to have 

 attained to full perfection. " It would appear," 

 says Dr. Arthur Farre, " as if muscular fibre 

 were here reduced to its simplest condition. 

 The filaments are totally disconnected, and are 

 arranged one above the other in a single series. 

 They pass straight and parallel from their ori- 

 gin to their insertion, and have a uniform dia- 

 meter throughout their course, except that each 

 filament generally presents a small knot upon 

 its centre, which is most apparent when it 

 is in a state of contraction, at which time the 

 whole filament also is obviously thicker than 

 when relaxed. The filaments have a watery 



* Dr. A. Fane, on the structure of Ciliobra- 

 chiate Polypi. Phil. Trans, part 2 for 1837. 



