I Scolecida. This division includes the parasitic worms (Entozoa), the Wheel-animalcules, and some allied forms, and is characterised by having an elongated or a flattened body, which may have an annulated integument, but which is not at all, or but imperfectly segmented. . A water-vascular system is present, but is not concerned with locomotion. There is no true blood-vascular system, and the nervous system consists of one or two cephalic ganglia, and never has the form of a gangliated ventral chain. Lateral appendages are almost universally wanting.
The Scolecida were formerly placed by Huxley along with the Echinodermata in a special sub-kingdom (Annuloida); and no doubt can be entertained as to the reality of the relationships between these two groups of animals. On the other hand, many and close points of affinity unite the higher Scolecida with the Ringed Worms (Annelida); and many syste-matists unite the Scolecida and Anarthropoda in a common sub-kingdom, to which they restrict the Linnean name of Vermes.
II. Anarthropoda. - This division includes the Spoon Worms (Gephyrea), the Ringed Worms (Annelida), and the Arrow Worms (Chaetognatha), and is characterised by the fact that the body is composed of a number (often indefinite) of similar or nearly similar segments arranged longitudinally. A "pseudo-haemaln system of vessels is generally present. The nervous system is placed ventrally, and consists typically of a double chain of ganglia, united by longitudinal commissures, and forming an oesophageal collar. Cilia are generally developed. Lateral locomotive appendages are usually present, but are never jointed or articulated to the body.
III. Arthropoda. - This division includes the Crustaceans, (Crustacea), the Spiders, Scorpions, etc. (Arachnida), the Centipedes and their allies (Myriapoda), and the Insects (Insecta). The body (fig. 112) is composed of a series (usually definite) of distinct rings or "somites," arranged along a longitudinal axis. A true blood-vascular system is normally present, and the heart is placed dorsally. The nervous system consists primitively of a double chain of ganglia, placed ventrally, and traversed anteriorly by the oesophagus. Limbs are almost always present, and are jointed and articulated to the body. The integument is more or less extensively hardened by the deposition in it of chitine, with or without salts of lime; and ciliated epithelium is not developed.
Fig. 112. - Diagram of the anatomy of an insect. an Antennae; e Eye; m Mouth; g Gullet; sg Salivary gland; s Stomach; f Tubes supposed to represent the kidneys ; i Intestine ; c Chamber (cloaca) into which the intestine opens ; v Vent; h Heart; n Nervous system ; l Bases of the legs.
The name of Scolecida was proposed by Professor Huxley * for the reception of the Rotifera, the Turbellaria, the Trema-toda, the Taeniada, the Nematoidea, the Acanthocephala, and the Gordiacea. Of these the Rotifera stand alone; whilst the Turbellaria, Trematoda, and Taeniada constitute the old division of the Platyelmia (Flat Worms); and the Nematoidea, Acanthocephala, and Gordiacea make up the old Nematelmia (Round Worms or Thread-worms). For some purposes these old divisions are sufficiently convenient to be retained, though they are of little scientific value. The term Entozoa has acquired such a general currency that it is necessarily employed occasionally, but it has been used in such widely different senses by different writers, that it would be almost better to discard it altogether. It certainly cannot be used as synonymous with Scolecida, many of these not being parasitic at all. It will therefore be employed here, in a restricted sense, to designate those orders of the Scolecida which are internal parasites, comprising the Trematoda, Taeniada, Nematoidea (in part), Acanthocephala, and Gordiacea. The Turbellaria and Rotifera, with a section of the Nematoidea, lead a free existence, and are not parasitic within other animals.
The Scolecida are defined by the possession of a "water-vascular system," consisting of a "remarkable set of vessels which communicate with the exterior by one or more apertures situated upon the surface of the body, and branch out, more or less extensively, into its substance" (Huxley). No proper vascular apparatus is present, and the nervous system (when present) "consists of one or two closely approximated ganglia" The body is not segmented, or but imperfectly so, and lateral appendages are absent in all except certain of the Rotifers. The habits and mode of life of the different members of the Scolecida are so different, that no other character, save the above, can be predicated which would be common to the entire group, and would not be shared by some other allied division. The most important morphological feature by which the Scolecida are separated from the Annelida, is that they are destitute of the ventral gangliated nerve-chain which is so characteristic of the latter group.
* More recently ('A Manual of the Anatomy of Invertebrated Animals,' 1877) Professor Huxley has abandoned the division of the Scolecida, and has separated its members into two sections (Trichoscolices and Nemato-scolices).