The classes and minor groups collectively termed Vermes do not constitute a phylum in any way comparable for example to the phyla Mollusca or Echinodermata. It is easy to show that the various Molluscan and Echinoderm classes are derived by a modification of a set of well-defined and peculiar characters. There is consequently amid much diversity a common thread running through the series. On the contrary the types of structure seen in most Vermian classes are very distinct from one another; they are specialised and therefore genetically remote. Attention nevertheless may be directed to certain general features.
The antero-posterior axis is usually of considerable length, the transverse and dorso-ventral short; but in the Myzostomidae and a few Polyclad Turbellaria the antero-posterior and transverse axes tend to an equality, and hence the body is disc-like. A ventral aspect is always distinguishable from a dorsal except in Acanthocephala; it is usually characterised by being flattened for locomotor purposes: sometimes also by the presence of various apertures, e.g. mouth, anus, etc, or of the main nervous cords. Bilateral symmetry is well marked, but a more or less apparent radial symmetry is observable in some Polyclad Ttirbellaria 1. A division of the body into somites is distinct in Chaetopoda and Archi-Annelida; present but obscured by a secondary annulation of the somites in Hirudinea; probably aborted in Gephyrea. It is indicated by the presence at regular intervals of bundles of dorso-ventral muscle-fibres, etc, in Nenter tea and Triclad Turbellaria. The Enteropneusta retain a division of the body into two regions, a collar and trunk, in correspondence with the mode of development of the coelome. The segmentation of the strobila in Cestoda is seemingly correlated solely with reproductive necessities.
Other Vermes are certainly uni-segmental. A large prae-oral lobe or prostomium is present in Enteropneusta and Gephyrea chaetifera. The corresponding region in Chaetopoda, Archi-Annelida, and Hirudinea is much reduced, and may in some cases be an outgrowth of the peristomial segment.
1 Much stress has lately been laid on the Ctenophoran characters of certain Polyclad Turbellaria. The presence of a free dorsally placed mass of otoliths in two genera, and of Ctenophore-like ciliary plates in one of the two, are striking features. But the existence in Turbellaria of a well-developed excretory system, the confinement of the principal nerve-cords to a ventral plane even when their radial symmetry is most marked, are features decisive against any close alliance with Ctenophora.
The coelome may be large, and is sometimes divided into compartments in segmented Vermes; it may be represented by irregular passages between the mesoderm cells, or even absent (?) altogether as in Acoelous Turbellaria. In the Hirudinea it undergoes obliteration (diacoelosis) to a very great extent during growth, and may be partially replaced by spaces secondarily formed (metacoelosis) in the mesoderm. It opens externally by special pores in many oligochaete Chaetopoda. In the Enteropneusta and Chaetognatha it is an enterocoele developed from the archenteron; in the segmented Vermes, and probably in Nematoda, it is a schizocoele; and in Rotifera it appears to be a permanent archicoele. The coelomic channels of Trematoda, etc, are apparently intercellular spaces 1.
The characters of the integument and disposition of the muscles of the body-wall are subject to much variation. It may be noted that the ectoderm, hypodermis or epidermis cells are transformed into the cuticle of Trematoda. It is uncertain whether or not the same is the case in Cestoda and Acanthocephala, but in the first-named the hypodermis is perhaps represented by sub-cuticular cells.
The nervous system may retain a position in the hypodermis ( = ectoderm) as in Enteropneusta, some Chaetopoda, Archi-Annelida, Gephyrean Priapu-lidae, some Nemertea, Chaetognatha, and to a certain extent in Nematoda. In the Turbellaria the central ganglia do not always bear any direct relation to the mouth or pharynx; and in Nemertea the commissures of the cephalic ganglia surround the proboscis and not the oesophagus. A peripharyngeal union of the cephalic ganglia is exceptionable in Trematoda. The ventral nerve-cords are segmented more or less distinctly in those Vermes in which the segmentation of the body is pronounced. Numerous longitudinal nerve-trunks of almost equal importance are found in Trematoda, some Polyclad Turbellaria and Nematoda. An almost radial symmetry in the Turbellaria in question is noticeable, but the ventral position of the trunks destroys an otherwise apparent resemblance to Ctenophora. Complete circular nerve-rings connecting the longitudinal trunks from place to place are present in some Gephyrea, Trematoda, and Nematoda. The predominance of the dorsal nerve-cord and the great development of a diffuse sub-epidermic nerve-layer in Enteropneusta, the varying positions of the two lateral cords in Nemertea, i. e. lateral, more ventral or more dorsal, and their occasional supra-anal union, are points bearing on the supposed relationship of these groups to Chordata. Organs of special sense occur in the shape of sense-cells with sense-hairs, aggregated into remarkable lateral sense-organs in the Chaetopod family Capitellidae; of sensory papillae, of eyes and otocysts.
But the last-named are far from common.
1 Fraipont's view as to the existence of intercellular coelomic spaces or channels in Cestoda and Trematoda is opposed by Pintner, who holds that, in some Cestoda at least, there is a system of intracellular canals with which the basal processes of the flame-cells are connected. The former view seems more likely to prove correct.
A digestive tract is not even indicated in development in Cestoda and Acanthocephala, probably in consequence of a long continued ancestral endo-parasitic life dependent on a supply of ready digested food. It is rudimentary in most male Rotifera, and in the sporocyst of Trematoda. It is replaced by the central parenchyma of the body, into which food passes through the mouth in the Acoela among Turbellaria. The form of the tract varies. Sacculation more or less distinct occurs in segmented Vermes. There is no anus in Trematoda or Turbellaria, and, what is more, there is no evidence to show that it has become aborted. When present it is terminal, ventral and sometimes dorsal (some Chaetopoda, Sipunculid Gephyrea, Hirudined). It is primitively dorsal in the larval Balanoglossus Koivaleivskii (Enteropneustd), but becomes terminal by the abortion of the sub-anal portion of the body. The existence of laterally placed respiratory apertures opening from without into the anterior part of the digestive tract may, among other features, indicate a connection between the Enter opneusta and the ancestral forms of the Chordata.