Most Chætopoda deposit their eggs before development. The Oligochæta lay them in peculiar cocoons or sacks formed by a secretion of the integument. Some marine Polychæta carry them about during their development. Autolytus cornutus has a special sack on the ventral surface in which they are hatched. In Spirorbis Pagenstecheri they develop inside the opercular tentacle, and in Spirorbis spirillum inside the tube of the parent. A few forms (_e.g._ Eunice sanguinea, Syllis vivipara, Nereis diversicolor) are viviparous. Perhaps the most primitive type of Chætopod development so far observed is that of Serpula (Stossich, No. 357)[135]. There is a regular segmentation resulting in the formation of a blastosphere with a central segmentation cavity. An invagination of the normal type now ensues. The blastopore soon narrows to become the permanent anus, while the invaginated hypoblast forms a small prominence with an imperfectly developed lumen, which _does not nearly fill up the segmentation cavity_ (fig. 139 A). The embryo, which has in the meantime become completely covered with cilia, now assumes more or less the form of a cone, at the apex of which is the anus, while the base forms the rudiment of a large præ-oral lobe. The alimentary sack grows forwards and then bends upon itself nearly at right angles, and meets a stomodæal invagination from the ventral side some way from the front end of the body. [135] The observations of Stossich are not thoroughly satisfactory. [FIG. 139. TWO STAGES IN THE DEVELOPMENT OF SERPULA. (After Stossich.) _m._ mouth; _an._ anus; _al._ archenteron.] The alimentary canal soon differentiates itself into three regions (1) oesophagus, (2) stomach, and (3) intestine. With these changes the larva, which in the meantime becomes hatched, assumes the characters of a typical Annelid larva (fig. 139 B). In front is a large præ-oral lobe, at the sides of which the eye-spots soon appear. The primitive segmentation cavity remains as a wide space between the curved alimentary tract and the body walls, and becomes traversed by muscular fibres passing between the two. The original chorion appears to serve as cuticle, and is perforated by the cilia. The further changes in this larval form do not present features of general importance. A peculiar vesicle, which in anomalous cases is double, is formed near the anus. If it were shewn to occur widely amongst Chætopoda, it might be perhaps regarded as homologous with the anal vesicles of the Gephyrea.