The atrochal larvæ are not common. The following history of an Eunicidan larva (probably Lumbriconereis) from Claparède and Metschnikoff (No. [336]) will illustrate their general history.
In the earliest stage noticed the larva has a spherical form, the præ-oral lobe not being very well marked. In the interior is a globular digestive tract. The cilia form a broad central band leaving free a narrow space at the apex of the præ-oral lobe, and also a circumanal space. At the apex of the præ-oral lobe is placed a bunch of long cilia, and a patch of cilia also marks out the anal area.
As the larva grows older it becomes elongated, and the anterior bunch of cilia is absorbed. The alimentary canal divides itself into pharynx and intestine. The former opens (?) by the mouth in the middle of the central band of cilia, the latter in the anal patch. The setæ indicating the segmentation are formed successively in the posterior ring-like area free from cilia. The cilia disappear after the formation of two segments.
In Lumbricus, the embryo of which ought perhaps to be grouped with the atrochæ, the cilia (Kleinenberg) cover a ventral tract of epiblast between the two mesoblastic cords, and are continued anteriorly to form a circle round the mouth.
The monotrochal larvæ are provided only with the important præ-oral ciliated ring before mentioned. In the majority of cases they are transitional forms destined very shortly to become telotrochal, and in such instances they usually have a more or less spherical body which is nearly divided into two equal halves by a ciliated ring. In some few instances, such as Polynoe, Dasychone, etc., the monotrochal characters are not lost till the larval cilia are exuviated.
The telotrochal forms (of which examples are shewn in [figs. 144], [150], etc.) may (1) start as monotrochal; or (2) from the first have a telotrochal character; or (3) be derived from atrochal forms. The last mode of origin probably represents the ancestral one.
Their mode of development is well illustrated by the case of Terebella nebulosa (vide Milne-Edwards, No. [347]). The embryo is at first a nearly spherical ciliated mass. One end slightly elongates and becomes free from cilia, and, acquiring dorsally two eye-spots, constitutes a præ-oral lobe. The elongation continues at the opposite end, and near this is formed a narrow area free from cilia. The larva now has the same characters as the atrochal Eunicidan larva described above. It consists of a non-ciliated præ-oral lobe, followed by a wide ciliated band, behind which is a ring-like area free from cilia; and behind this again a peri-anal patch of cilia. The ring-like area free from cilia is, as in the Eunicidan larva, the region which becomes segmented. It soon becomes longer, and is then divided into two segments; a third and fourth etc. non-ciliated segment becomes successively interposed immediately in front of the peri-anal patch; and, after a certain number of segments have become formed, there appear on some of the hinder of them short tubercles, provided with single setæ (the notopodia), which are formed from before backwards, like the segments.
The mouth, anus, and intestine become in the meantime clearly visible. The mouth is on the posterior side of the ciliated band, and the anus in the centre of the peri-anal patch.
The ciliated band in front now becomes contracted and provided with long cilia. It passes below completely in front of the mouth, and constitutes, in fact, a well-marked præ-oral ring, while the cilia behind constitute an equally marked peri-anal ring. The larva has in fact now acquired all the characters of a true telotrochal form.
Only a comparatively small number of Chætopod larvæ remain permanently telotrochal. Of these Terebella nebulosa, already cited (though not Terebella conchilega), is one; Polygordius, Saccocirrus and Capitella are other examples of the same, though in the latter form the whole ventral surface becomes ciliated.