Lastly, in the Siponculi the vermiform ap pearance is completely established, the longi tudinal and circular muscles that bound the visceral cavity are strongly and distinctly deve loped, the complicated apparatus of foot-like suckers has disappeared, minute ganglia are visible towards the anterior end of the body, and we arrive at the annulose condition, that characterizes the next great division of the animal creation, which now offers itself to our contemplation.
110MOGANGLtATA (Owen).—The third great natural group of living beings consists of crea tures having the exterior of their bodies divided into rings or segments arranged behind each other in a longitudinal series, and generally furnished with lateral appendages of different kinds symmetrically disposed, which are sub servient to many and very various purposes. The nervous system, moreover, throughout the entire range of this extensive series assumes a new and constant arrangement in itself, quite sufficient to characterize this sub-kingdom of animated nature, and with the different modifi cations of this portion of their economy are intimately connected the progressive changes observable in the structure and habits of the different classes included therein.
In the simplest conceivable condition under which a Ilomogangliate animal could exist, and doubtless among the lowest of the red-blooded worms and most imperfect forms of insect larvm such a condition might be pointed out, the body would consist of a long succession of similar rings, each of which would contain an appropriate nervous apparatus consisting of a pair of ganglia symmetrically disposed on each side of the mesian line, from which nerves proceed for the innervation of the segment in which the brains or ganglia were placed. These ganglia in each segment communicate with each other and likewise with the pairs that precede and follow them by inter-communicating ner vous filaments, and thus the entire series of individual brains or ganglia is united into one system made up of as many pairs as there would be rings entering into the composition of the body. There is, however, a remarkable difference between the anterior pair of ganglia and those placed in the succeeding segments. The first pair is invariably situated above the cesophagns on its dorsal aspect, while all the rest are arranged beneath the alimentary canal along the ventral region of the animal, so that the nervous cords that join the first and second pairs of ganglia embrace the oesophageal tube. The supra-a'sophogeal pair of brains invariably communicates with the instruments of the senses whenever such exist, and therefore is very justly comparable to the encephalon of higher animals; while the succeeding chain of sub- esophageal ganglia animate the muscles of the different segments of the body, and may therefore be looked upon with great reason as representing the spinal cord of the Vertebrata.
But while the ganglia either of the head or of the ventral cord are thus numerous, as we have supposed them to be, in the lowest worm, they are as yet by far too small and devoid of energy in such a dispersed condition to corres pond with organs connected with the higher senses, or even to wield muscles of sufficient power to support the weight of the body raised on articulated limbs. Therefore, before senses can be given or active limbs bestowed, a pro cess of concentration must be gone through, the encephalic masses must be enlarged and thus rendered more perfect, the ganglionic centres that influence muscular movements reduced in number and made proportionately more ener getic, and exactly in the ratio in which this improvement is effected in the nervous system, do the muscles become by degrees stronger and more efficient, and the limbs appended to the body more active and useful as locomotive agents. This, however, will be best exempli fied by a rapid survey of the principal classes that compose the division of animals we are now considering.
In the lowest Annelidans, as for example the Gerdias or hair-worm, so impotent are the minute ganglia bestowed, that even the rings upon the exterior of the body are scarcely indicated, and not the least vestiges either of limbs, tentacula, or eyes are to be detected. In the Leeches even, although the number of ganglia is in them considerably diminished, and rings of the body more strongly marked, exter nal limbs cannot as yet be given, their place being supplied by the suctorial discs of the head and tail ; nevertheless, even in these aquatic Annelides, the encephalic masses are sufficiently advanced to permit organs of vision to be granted, and accordingly for the first time in the animal series (as far as our own belief goes) are real eyes met with. The muscular system in these humble worms consists exclu sively of the contractile walls of ihe body, the fibres of which are arranged in three strata, superposed one upon the other, and pass in different directions, one stratum being com posed of longitudinal, another of obliquely spiral, and another of annular fibres surround ing the body of the animal ; but these are sufficient for progression, all the movements of contraction, elongation, or flexure of the body being provided for by this simple arrangement.