234 PHYLUM ANNELIDA. 
There is a metamorphosis in development, but the nature of the larva 
differs markedly in the different genera. In Ech¢éwrus and Thalassema 
it bears a striking resemblance to a Trochosphere. Thus there is a 
well-developed pre-oral lobe with an apical sense organ, and pre-oral 
and post-oral bands of cilia. ‘‘ Head kidneys” or provisional nephridia 
occur, and the post-oral region shows distinct segmentation, the 
segments being marked externally by rings of cilia. As development 
proceeds, all trace of segmentation is lost. In Bonediza the larva shows. 
no trace of segmentation, and is Turbellarian-like ; owing to a premature 
arrest of development, the male remains at this level throughout life. 
Appendix (1) to Chetopoda 
Primitive Forms. ARcCHI-CHATOPODA or 
ARCHI-ANNELIDA 
There are a few, small, simple, marine worms, with some Annelid or 
Cheetopod characters, which are sometimes supposed to be ancestral 
forms. Thus Dzzophzlus is a minute Planarian-like animal found among 
Alge. In the young at least the body is distinctly segmented, but 
there are no bristles, gills, or tentacles. There are circling bands 
of cilia. The nervous system consists of a brain and two widely 
separated ventral ganglionated cords, but it remains in contact with the 
epidermis. 
More distinctly Annelid are the marine worms Polygordius, Proto- 
drtlus, Saccocirrus, and Histriodrilus. 
The small body is segmented and uniform; there are no sete, 
parapodia, cirri, or gills, but the head bears a few tentacles ; the pre- 
oral region is small, and the segment around the mouth is large ; the 
very simple nervous system is retained in the epidermis. 
Polygordius (Fig. 119 (11)) is a thin worm, an inch or more in length, 
living at slight depths in sand or fine gravel, often along with the 
lancelet. It has two tentacles, a few external cilia about the mouth in 
a pair of head-pits, and sometimes on the body; it moves like a worm, 
but has no bristles. It feeds like an earthworm, or sometimes more 
discriminatingly on unicellular organisms. The females are usually 
larger than the males, and in some species break up at sexual maturity. 
The development includes a metamorphosis, and the larva are 
ciliated, free-swimming, light-loving, surface animals, feeding on 
minute pelagic organisms, seeking the depths as age advances. 
According to some, the larva represents a primitive unsegmented 
ancestral Annelid, with medusoid affinities ; according to others, the 
larval characteristics are adaptive to the mode of life, and without 
historic importance. 
Protodrilus is even smaller than Polygordius, with more cilia, mobile 
tentacles, and two fixing lobes on the posterior extremity ; the move- 
ments are Turbellarian-like, the reproductive organs hermaphrodite, 
the development direct. ¢strdodrzlus is parasitic on the eggs of the 
lobster, and its affinities are doubtful. ‘ 
