[132] In the abstract in Hoffman and Schwalbe Kowalevsky is made to state that the tentacles spring from the border of the mantle. This can hardly be a correct account of what he states, since it does not fit in with the adult anatomy of the parts. The figures he gives might lead to the supposition that they sprang from the edge of the cephalic lobe, or perhaps from the dorsal lobe of the mantle.
[133] For the ectoproctous Polyzoa it might be held that the ciliated ring of tentacles is post-oral, but the facts of development recorded in the previous chapter appear to me to shew that this view is untenable.
CHAPTER XII.
CHÆTOPODA[134].
Formation of the Germinal Layers.
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.