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.
(705). In Polynoe cirrata the months of February and March are the period of propagation, when the body assumes a pale rose colour, arising from a numberless quantity of eggs, which fill the abdominal cavity, with the exception of about the first anterior fourth, and appear everywhere through the skin. When the animal is opened, the eggs appear to hang together in masses by means of a connecting tenacious mucus. In other individuals the eggs occur on the top of the back of the mother, beneath the dorsal scales, in immense numbers, surrounded by a tenacious mucus. The heaps of eggs cover the whole hinder half of the back, but, more anteriorly, only the sides above the bases of the feet. It would seem that the eggs pass out through a very small aperture just above the feet, as Rathke found in the case of Nereis pulsatoria. Here, protected beneath the dorsal plates, the eggs remain until the young escape. In the meantime the yelk undergoes the usual process of mulberry-fission, until it becomes finely granular. The ova become slightly oval; and the foetus (into which the entire yelk is converted, without any part whatever separating) is smooth, greyish white, and more or less narrowly enclosed in chorion.
A peculiar kind of motion was now observable on placing the separated ova under the microscope, the ova turning round and round: this was effected by a very short fringe, which is seen now and then to move slowly and curve in a worm-like form, drawing the egg with it backwards and forwards. The foetus itself, which gradually acquires a white greyish-green colour, is still without motion in most of the ova: only, in a few a circle of extremely minute projecting and vibrating cilia was perceptible, which surrounds horizontally the centre of the body of the foetus, at an equal distance from the two poles of the ovum. At last the foetus arrives at maturity, and the mother now carries on her back many thousands of young ones, which gradually come forth from the mucus surrounding the eggs, leave their mother, and swim freely about in the water, visible to the naked eye as very minute greenish-grey points (1/20th of a millimetre in size)
* Sars, Wiegm. Archiv, 1845, part 1.
(706). Many interesting particulars relative to the development of various genera belonging to the class under consideration have been ascertained by Milne-Edwards*. In the Terebellce (fig. 136), according to the observations of this distinguished naturalist, the young, on leaving the egg, have no resemblance whatever to the adult animal, insomuch indeed that it would be difficult to guess, a priori, the class to which they really belonged. On their first appearance upon the stage of active existence they might be mistaken for the ciliated larvae of certain Polyps or Medusae, presenting no traces of the annulose type of structure (fig. 136, l): in the course of a short time, however, their bodies become elongated, and they begin to assume somewhat of a bilateral or symmetrical form, the body of the young Terebella becoming distinguishable, divided into four zones or rudimentary segments, the posterior of which is still provided with a ciliary apparatus (fig. 136, 2.) Shortly after this, a fifth ring (fig. 136, 3, d) begins to make its appearance in the space situated between the penultimate and terminal, and rudiments of a mouth and alimentary canal become distinguishable. The growth of the young Annelidan now begins to advance rapidly; and its body is rendered more worm-like as new segments are progressively added to its length, these all successively making their appearance in the space between the last-formed ring and the anal or terminal joint of the body; so that the relative position of the newly developed segments is precisely in accordance with their respective ages, except in the case of the last segment, which is persistently terminal. Meantime the larva ceases to be, as it was at first, completely apodous: simple subulate setae, supported upon minute fleshy tubercles, begin to show themselves on both sides of the body, the development of these locomotive appendages being accomplished in the same order of sequence as that of the segments, namely from before backwards.
(707). At this period of their growth the young Terebellce present the appearance of minute subcylindrical worms (fig. 136, 4), and the different viscera in the interior of the body become very clearly defined.
* "Recherches Zoologiques faites pendant un Voyage sur les Cotes de la Sicile, par M. Milne-Edwards," Ann. des Sci. Nat, for 1844.
(708). The digestive apparatus is now distinctly perceptible: anteriorly it presents a kind of fleshy bulb (fig. 136, 4, p); then a short cylindrical oesophagus, followed by a capacious ovoid stomach (r), the contents of which appear to be still saturated with the coloured substance of the vitellus, and an intestine (s), which commences at about the posterior third of the body. The glandular structures near the anterior part of the animal now become apparent, and the subcutaneous muscles clearly distinguishable; still it is remarkable that, even in the most transparent portions of the creature, no traces of a vascular system can be detected.
(709). In the course of three or four days more, the cilia have completely disappeared from the surface of the body, which now presents all the characters of one of the erratic Annelids, but in no respect resembles the tubicolous genera to which the creature really belongs. The young larva, in short, is furnished with a distinct head, an antennary organ, eyes, and feet armed with subulate setae; while the adult Tere-bellae are acephalous, being destitute both of antennae and eyes, and having feet provided with hook-like appendages.
(710). After the larva has been furnished with one or two additional pairs of feet, the head begins to be changed in its shape (fig. 136, 5), a transverse constriction makes its appearance at a little distance in front of the eyes, and its anterior lobe, which thus becomes distinctly defined, is seen to be studded near its free margin with a series of stinging vesicles, some of which are armed with little spine-like filaments. The post-cephalic ciliated collar becomes at the same time much narrower, and forms a prominent ridge underneath the head, that constitutes a kind of upper lip. In the course of two or three days more, the anterior cephalic lobe (136, 5, a) becomes perfectly distinct from the oculiferous segment, and is much elongated, taking a cylindrical form, and constituting a very flexible median appendage, having all the characters of an antenniform organ. Its axis is occupied by a canal that communicates with the general cavity of the body; and a fluid may be seen to circulate in its interior. The natatory cilia have almost entirely disappeared both from the neck and from the posterior extremity of the body; and the young Terebella in this condition presents itself exhibiting all the characters of an Annelid belonging to the erratic group-not, as yet, at all resembling any of the tubicolous genera, of which it is a member.