WORMS, or VERMES (COMPARATIVE ANATOMY). Most zoologists regard the worms as constituting a subdivision of the ARTICULATA; but one of our latest and best writers on classification—prof. Huxley—confines the synonymous terms ARTICULATA and AR TIIROPODA to the insects, myriapods, arachnidans, and crustaceans; and places the higher worms, or annelids, with the above classes, in a primary division, or subkingdom, of ANNULOSA; and the less highly organized worms, scolecids (in which he includes the rotif era or wheel-animalcules, the trematoda or flukes, the teenia,da or tapeworms, the nema toidea or thread-worms, the acanthocephala and the gordiacea), in a subkingdom, to which he applies the term ANNULOIDA. The main reasons of his placing the worms under two great subdivisions are—(1) that the annelids resemble the arthropoda in the arrangement of the nervous system, which constitutes a ganglionated double chain, traversed at one point by the oesophagus; (2) none of the seolecids possess any characters in common with the arthropoda generally, or the annelids, other than those which they have in common with all animals. No scolecid has a definitely segmented body, or bilaterally disposed successive pairs of appendages, nor has it a longitudinal chain of ganglia. These grounds of difference outweigh, in his opinion, the many points of resemblance between the annelids and the scolecids—as (1) the resemblance between the ciliated larvae in many cases ; (2) the resemblance between the forms of the mature bodies of many scolecids with that of one of the most familiar of annelids, which is so close as to have acquired for the scolecids the popular name of "worms;" and (3) the fact that in the annelids we see the representatives of that singular system of vessels which attains a perfect development in the " water-vascular" apparatus of many scolecids. The final settlement of the classifica tion of these animals must be decided by further investigation.
With regard to the general characters of worms, it is well known that they are usually of a very elongated form. In the higher groups the division of the body into a number of segments is very distinct; while in some of the lower forms no segmentation can be detected. The segments, when present, are usually homonomous, or, in other words, are mere repetitions of one another. The soft and contractile body may be cylindrical or slightly compressed, or it may be flat and broad, and usually presents a distinct dor sal and abdominal surface. The lateral region is often provided, in the higher forms, with special appendages, resembling minute stumps, which take part in the respiratory process. Among the cuticular appendages must be mentioned the bristles (seta), hairs, hooks, etc., which are often seen. The nervous system of the highest worms—the annelids—has been already sufficiently described in our notice of prof. Huxlev's views. From this condition it appears in the seoleeids to become more and more rudimentary, till in the parasitic worms it totally disappears: The mouth is absent in the lower forms, but in the higher lies in the mesial line of the abdominal surface, in close approxima tion to the chief nervous (pre-oral) ganglion, from which most of the organs of the senses derive their nerves, as the eye, the auditory apparatus, and the organs of touch (especially the lips). Some of the parasitic worms, as the tapeworms, etc., are totally
devoid of an intestinal canal; others, as the turbellaria (with few exceptions), and the trematoda, have an intestine, but no anal aperture; while the rest have an intestine provided with both mouth and anus. The fatter, when present, lies on the posterior part of the body, and sometimes (as in many turbellaria) on.the dorsal surface. Except in the gephyrea or sipunculacea, the intestine, when present, is simple, and devoid of convolutions; but is often, as in the leech, provided with lateral blind sacs. The vascu lar system in the most highly organized worms consists of a closed system of arteries and veins, presenting modifications in different genera. A large vessel which runs beneath the dorsal integument may be seen under a microscope to contract and propel the blood forward, thus fulfilling the functions of a heart, and being the homologue of the dorsal vasiform heart of insects; while a corresponding venous trunk conveys the blood in an opposite direction, and runs along the under surface of the body. These great trunks are united at each segment by transverse vessels, which carry the blood from the ventral vein to the dorsal artery. In the nematelmia, or parasitic round-worms, the system is much simpler; and in the lowest worms no trace of true blood-yessels is discernible. None but the annelida (q.v.), or highest worms, possess special respiratory organs. These occur in various forms. Thus, in the leech and earthworm, a series of pores on each side of the body lead to as many simple saceuli formed by an inward folding of the integument. In the tubicolous annelids, such as the serpula (a common inhabitant in the aquarium), the respiratory organs are in the form of long flattened branchim, radiating from the head, and generally disposed in a spiral form. When not filled by the red circulating fluid which the annelids generally possess, they are often beautifully tinted with purple, green, and yellow colors, and form a gorgeous crown. In the arenicola piseatorum (figured in the article ANNELIDA), the respiratory organs are seen lying as lateral tufts in the middle part of the body (14 or 16 in number on each side). In the lower worms, there are no definite respiratory organs, the process being carried on partly by the surface of the skin generally, and partly by the water-canals noticed in the article TAPEWORM. As a general rule the worms are hermaphrodites, only one of the five classes into which they are divided—viz., the nematelmia, having the sexes separate. A,large number of the lower kinds are parasitical; the others are inhabitants of sea and fresh water, mud, earth, etc.