In the higher Annelida each segment (fig. 126) consists of two arches, termed, from their position, respectively the "dorsal arc" and the "ventral arc;" and each bears two lateral processes, or "foot-tubercles" (parapodia), one on each side. Each "foot-tubercle" is typically double, being composed of an upper process, called the "notopodium," or "dorsal oar," and a lower process termed the "neuropodium," or "ventral oar;" but these may be fused together. The foot-tubercles,likewise, support bristles, or "setae," and a soft cylindrical appendage, which is termed the "cirrhus" (fig. 126).
Fig. 126. - Diagrammatic transverse section of an Annelide. d Dorsal arc; v Ventral arc; n Branchiae; a Notopodium, or dorsal oar; b Neuropodium, or ventral oar, both carrying setae and a jointed cirrhus (c).
The number of the segments varies much, being as many as 400 in Eunice gigantea ; and, generally, there is not a distinct head which is separable from the succeeding rings of the body. When such a distinct head appears to be present, it is not comparable with the head of the Arthropoda, but is really a greatly modified prae-oral region, or "prostomium," as is shown by the position of the mouth. The "prostomium" or "cephalic, lobe" is placed in front of the mouth, and often carries "tentacles" above and tactile processes or "palpi" below.
The digestive system of the Annelides consists of a mouth, sometimes armed with horny jaws, a gullet, stomach, intestine, and a distinct anus. Except in the Hirudinea, the alimentary canal is suspended in a capacious perivisceral space, divided into compartments by more or less complete partitions. The alimentary canal is, with one exception, not convoluted, and extends straight from the mouth to the anus ; but lateral diverticula are often present.
As regards the vascular system, " no Annelide ever possesses a heart comparable to the heart of a Crustacean or Insect; but a system of vessels, with more or less extensively contractile walls, containing a clear fluid, usually red or green in colour, and in some cases only corpusculated, is very generally developed, and sends prolongations into the respiratory organs, when such exist" (Huxley). This system has been termed the "pseudohaemal system," and its vessels are considered by Professor Huxley as being "extreme modifications of organs homologous with the water-vessels of the Scolecida:" since the perivisceral cavity, with its contained corpusculated fluid (chyl-aqueous fluid), is, as shown by M. de Quatrefages, the true homologue of the vascular system of Crustacea and Insects. The pseudohaemal system, therefore, of the Annelides is to be regarded as essentially respiratory in function. The pseudohaemal vessels are sometimes wanting, and in these cases respiration appears to be effected by the cilia lining the perivisceral cavity.
Respiration is effected by the general surface of the body, or by distinct external gills or branchiae.
The excretory organs of the Annelides are the so-called "segmental organs." In their simplest form (as in the ordinary Leeches), each segmental organ is in the form of a much-folded tube, partly labyrinthic, partly vesicular, often with an appended caecum, and opening externally by a distinct aperture or "stigma," but having no internal communication with the body-cavity. In these cases, the segmental organs may be regarded as representing the kidneys of the higher animals.
In the higher Annelida, the segmental organs are usually in part subordinated to the function of reproduction. In these cases (fig. 127) the inner surface of the convoluted tube, which constitutes the segmental organ, is ciliated ; and the tube not only opens exteriorly by a distinct "stigma," but also communicates internally with the perivisceral cavity by a widely patulous, trumpet - shaped, internally-ciliated in-fundibulum (fig. 127, i), by which the products of generation are taken up and conveyed to the outer medium. Very usually, also, there are appended to the tube of the segmental organ blind glandular pouches, which represent the kidneys, or in other cases caecal appendices (fig. 127, s) for storing up the generative products.
The nervous system consists of a double, ventral, gangliated cord, which is traversed anteriorly by the oesophagus; the "prae-cesophageal," or "cerebral," ganglia being connected by lateral cords or commissures with the "post-oesophageal" ganglia. Pigment-spots, or "ocelli," sometimes of high organisation, are present in many, generally upon the proboscis, sometimes in each segment, or on the branchiae, or on the tail; and the head often supports two or more feelers, which differ from the "antennae" of Insects and Crustacea in not being jointed.
The sexes in the Annelida are sometimes distinct, and sometimes united in the same individual. The embryos are almost universally ciliated, and even in the adult cilia are almost always, if not always, present - in both of which respects this class differs from the Arthropoda.
The Annelida may be divided into two sections, characterised by the presence or absence of external respiratory organs or branchiae. The Abranchiate section comprises the Leeches and the Earth-worms; whilst the Branchiate division includes the Tube-worms (Tubicola) and the Sand-worms (Errantid). The Annelida are also often divided into two sections, called Chaeiophora and Discophora, according as locomotion is effected by chitinous setae (Earth-worms, Tube-worms, and Sand-worms) or by suctorial discs (Leeches).
Fig. 127. - Segmental organ of of a Chaetopodous Annelide (Alciopa), enlarged. o External aperture or stigma; t Tubular and ciliated portion of the segmental organ; s Seminal receptacle; i Ciliated infundibulum opening into the body-cavity. (After Claparede.)