They divide accordingly into three orders, and each of these again into three families, in accordance with the classes, namely, those of the Protozoa or Mucus-animals, through which they pass.
Order 1. Infusorial Worms—Parasites.
3504. The White-blooded Worms or Entozoa are very imperfect animals, a fact which proves that this circle recommences quite inferiorly or from below. They live for the most part in the interior of other animals, being thus located in darkness and in places where they obtain but little oxygen. Their blood therefore is not simply colourless, but even its vessels are only imperfectly developed. They respire without doubt through the tegument. In many, a distinct or separate intestine being wanting, it is the tegument also which digests; in others the intestine is simply a sac without an anus. The sexual parts too are in many species of a doubtful character, while meantime there are both androgynous and diœcious individuals. In the latter the male parts open invariably at the posterior extremity of the body, as in Insects; the female parts in front of its caudal end, as in the Crustacea. Both are in other respects constructed as in Insects, namely, there are two oviducts or seminal ducts, which unite before reaching their external orifice. They divide into three groups.
In one of these the body is tolerably smooth, and the pharynx commencing from a simple suctorial mouth elongates into a ramified intestine without an anus; they are androgynous—Saugwürmer.
In others the body itself supplies the place of the stomach; it is corrugated, and receives the food through one or several orifices, without separating into a special intestine. They have nearly all a claviform proboscis or rostrum, with which they perforate substantial cavities for themselves; it would seem that they are both androgynous and of separate sexes—Hydatids and Tæniæ.
Others are separated into tegument and intestine like the Mussels and Snails, but without having a distinct vascular system, heart, and liver, though provided with a nervous chord and separate sexual parts—Ascarides.
Fam. 1. Monad-like Worms, Saugwürmer.
Body tolerably smooth, suctorial mouth and sucker, the intestine losing itself in the tissue of the body, and without an anus; androgynous.
They remind us through their small size and structure of the Monades, especially the Cercariæ, and among the Worms typify or prefigurate in particular the Hirudines or Leeches, both by their form and power of suction, as also in the ramification of the intestine.
Many Cercariæ might be metamorphosed into Distomata, and so be or constitute their young.