Fig. 146. Polygordius larva.
(From Alex. Agassiz.)
At the sides of the head there is formed a pair of ciliated pits, also found by Hatschek in the embryo of Criodrilus, and characteristic of many Chætopod larvæ, but persistent in the adult Polygordius, Saccocirrus, Polyophthalmus, etc. They are perhaps the same structures as the ciliated pits in Nemertines.
Fig. 147. Polygordius larva. (From Alex. Agassiz.)
During the external changes above described, by which the adult form of Polygordius is reached, a series of internal changes also takes place which are for the most part the same as in other Chætopoda; and do not require a detailed description. The nervous[138] and muscular systems have precisely the normal development. The division of the mesoblast into somites is not externally indicated. The organs most worthy of notice are the excretory organs.
The essential points in the above development of Polygordius are (1) the gradual elongation and corresponding segmentation of the post-cephalic part of the body; and (2) the relative reduction in size of the præ-oral lobe and its conversion together with the oral region into the head; (3) the atrophy of the ciliated bands. The conversion of the larva into the adult takes place in fact by the intercalation of a segmented region between a large mouth-bearing portion of the primitive body and a small anus-bearing portion[139].
