This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(408). In Planaria tremellaris, the penis, which during copulation is protruded from the anterior orifice (fig. 75, 6), is a white contractile body, enclosed, when in a retracted state, in a small oval pouch; it is perforated with a minute canal, and receives near its root two flexuous tubes, which gradually decrease in size as they diverge from each other, until they can no longer be traced. These are the seminiferous vessels (fig. 75, 5, a.) The posterior genital orifice, which leads to the female organs, communicates with a small pouch, or uterus, as it might be termed (fig. 75, 5, b); into this open two lateral oviducts, which run on each side of the male apparatus and of the proboscis; these are very transparent, and only recognizable under certain circumstances by the ova which they contain. In Planaria lactea the oviduct opens into the uterine cavity by a single tube, which, passing backwards, divides into two equal branches; and both of these, again subdividing, ramify extensively among the caeca derived from the stomach. We likewise find in this species two accessory vesicles, which pour their secretions into the terminal sac.
(409). The Nemertean Helminthozoa are marine animals, frequently found lurking beneath stones, or in clefts of rock, on shores left by the retreating tide. In their external appearance they have some resemblance to Taeniae, and, like them, are occasionally met with of prodigious length. In their anatomical structure, however, they differ widely from the Tape-worms, as will be immediately evident from the following details.
(410). The body is covered with a sort of semifluid varnish, the whole surface of which is densely clothed with extremely delicate and closely set vibratile cilia, so minute as to be perceived with difficulty, except in the vicinity of the head, where, at certain points, they present more conspicuous dimensions (fig. 76, b b).
(411). The alimentary apparatus presents a remarkable uniformity of arrangement, and, for the purpose of description, may be divided into the mouth, the proboscis, the oesophagus, and the intestine. There do not appear to be either salivary or hepatic glands; neither is the intestinal canal provided with an anal outlet. The mouth is a minute orifice, situate at the anterior extremity of the body (fig. 76, a), and generally surrounded with cilia of large dimensions, from which a muscular canal is prolonged for some distance backwards, at the posterior extremity of which may be seen, folded up in the interior of the body, a powerful proboscis, which is protrusible at the will of the animal, and, like that of the Planaria described above, is a most efficient instrument for seizing prey. Immediately behind the proboscis is situated a very remarkable apparatus in the shape of an extremely muscular oesophagus, wherein is lodged a weapon of most extraordinary character, consisting of a solid transparent spiculum, lodged in a cavity, formed in the thickness of the oesophageal parietes, wherein is contained a glandular mass, by which the calcareous spiculum is apparently secreted (fig. 77, a.) Other glandular structures (h h), probably destined for the elimination of some venomous secretion, are in communication with the styliferous cavity; so that the dagger-like spiculum, being constantly bathed with the secretion thus furnished, becomes an instrument of a very deadly character. The dagger-like weapon, together with its immediate appendages, is capable of being protruded by the aid of muscular bands in connexion with the styliferous cavity, which at the same time, by compressing the glands, readily cause the emission of their secretion. On each side of the formidable apparatus above described, there are, moreover, two other cavities, apparently of an auxiliary character (fig. 77, i i), in which a secretion of supernumerary spicula is perpetually in progress, apparently destined to replace the original one if lost or broken.
Fig. 76. Anterior portion of Borlasia camillea. a, buccal orifice; b b, cephalic fossa, provided with vibratile cilia; c c, lobes of the brain, brought into communication with each other by means of a broad sub-ceso-phageal commissure, and apparently composed of several ganglia conjoined; d d, longitudinal nervous trunks, whence branches are derived which supply the muscles and internal viscera; e e e e, cephalic nerves; f f, groups of eye-like specks; gg, cephalic loop of the vascular system; 11, medio-dorsal vessel, which, on entering the cephalic cavity, bifurcates to form the trunks, k k, that encircle the brain, and afterwards, at h h, become continuous with the lateral vessels,i i; m m mm,horizontal diaphragm, forming a sheath, in which is lodged the anterior portion of the proboscis, o o; n n, the ovaria (testes in the male), furnished with caeca that float freely in the lateral chambers of the body. The space comprised between the septa, to which the generative organs are attached, forms the central longitudinal cavity wherein is lodged the digestive apparatus.
(412). The intestine (fig. 77, b), properly so called, succeeds immediately to this strangely organized oesophagus, and is continued backwards, floating in the central compartment of the body,to which it is loosely attached by membranous bridles for about three-fourths of the length of the worm, at which point its calibre rapidly diminishes and its cavity soon becomes obliterated, so that it terminates under the aspect of a simple cord.
(413). The Nemertean Helminthozoa are possessed of a very complete circulatory system of vessels enclosed in proper walls. These consist of three principal trunks, two of which run along the sides of the ventral aspect of the body (fig. 76, i i), while the third occupies the middle of the dorsal surface (fig. 76, I I): these three vessels become considerably increased in size as they approach the posterior extremity of the worm, and ultimately are conjoined.