NEMERTINEA (NEMERTINES or NEMERTEANS), a group of unsegmented worms showing considerable resemblances to the Platyhelminthes (q.v.), but generally looked upon as a distinct phylum. Almost all nemertines are free-living, and the majority are marine, though a few live in fresh water or even on land. Their most characteristic feature is the possession of a remark able eversible proboscis lying in a special sheath dorsally to the alimentary canal.
The proboscis pore (or rhyn chostome) and the mouth may have a common opening at the anterior end, or the mouth may be distinct and situated subter minally on the ventral surface. There is an intestine opening posteriorly by an anus, and sometimes preceded anteriorly by a specialized oesophagus or "stomach" with muscular walls. In many species the intestine has paired lateral pouches, and an appearance of metameric segmentation is produced by the pres ence of a series of transverse sheets of muscular fibres stretching across the body and separating each pair of pouches from its neighbours. In certain forms there is an anterior, ventral, blind prolongation of the intestine below the "stomach." This has been shown to open in the embryo as the blastopore, and subsequently to lose its connection with the exterior.
The proboscis forms a blind invagination lying loosely within the proboscis sheath, or rhyn chocoele. Its anterior portion can be everted by the contraction of the muscular walls of the sheath, and the consequent pressure of the fluid which this contains. The posterior portion is pre vented from being everted by a retractor muscle attached to the wall of the rhynchocoele. The proboscis itself is generally very muscular and richly supplied with nerves. In some forms it is unarmed and probably mainly sensory in function. In others, however, it contains, in that region which becomes the ante rior end in the everted condition, one or several calcareous stylets mounted on a special base, in close relation with a muscular bulb and a duct which may serve for the expulsion of a "poison." In the walls of the proboscis in armed forms, near the base of the stylet, there are two or more pockets containing "reserve" stylets. These probably take
the place of the functional stylet if this is broken off, but how this occurs is unknown. In some forms the lining of the pro•. boscis is said to contain "nematocysts" or stinging-cells like those of the coelenterates.
The central nervous system, or brain, consists of a ring sur rounding the proboscis-sheath anteriorly, and developed into lateral masses or lobes, of which there are usually two dorsal and two ventral. The brain gives off two main lateral nerve cords which run throughout the length of the body posteriorly, and sometimes unite dorsally near the anus. It also sends a pair of large nerves to the proboscis, a pair of nerves to the sense organs of the anterior region, and a pair to the oesophagus. There is, in many forms, a plexus of nervous tissue between the muscu lar layers of the body wall. In others this is absent, but the various parts and organs of the body are innervated by branches from the lateral cords. In asso ciation with the brain there is usually a pair of lateral cephalic pits or grooves, containing mu cus-secreting cells and connected by ciliated canals with special masses of nervous tissue. The function of these organs is un certain. A pair of shallow, ever sible, lateral pits, probably sen sory, also occurs farther back on the body in certain species. In many forms eyes are present, and these organs may be very numerous. They may be mere spots of dark pigment, or may be provided with a cellular lens.
There is a closed circulatory system, containing a colourless fluid and nucleated corpuscles which in some species appear to contain haemoglobin. The main vessels are three longitudinal trunks, one median and two lateral. These are connected at both ends of the body. At the anterior end, in some forms, they break up into a system of sinuses, while in others they are joined by a simple loop-like commissure. A series of metamerically arranged transverse vessels is present in many forms, connecting the three longitudinal vessels along their course.
The excretory organs, when present, usually consist of a single pair of nephridia opening laterally by ciliated ducts a little be hind the mouth. Their inner ends are branched and terminate in a number of "flame-cells" closely applied to the walls of the lateral blood-vessels in the anterior region.
The sexes in the Nemertinea, with few exceptions, are separate. The gonads are constructed on a very simple plan, in strong con trast with the complicated system usually met with in the Platy helminthes. They consist of a series of simple sacs, alternating with the lateral pouches of the intestine, and opening, when ma ture, by separate ducts to the exterior. The sexual products (ova or spermatozoa) are proliferated from the lining epithelium of the sacs. It is probable that the cavities of these sacs represent the coelom.
Classification.—The modern systematic arrangement of the Nemertinea is based chiefly on the armed or unarmed condition of the proboscis, the arrangement of the muscle-layers and the position of the nerve-cords. The group is divided into two sub classes and four orders, as follows:—