270 PHYLUM ANNELIDA 



Echiurus and Thalassema it bears a striking resemblance to a Trocho- 

 sphere. 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 Bonellia 

 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 (i) to Chcetopoda 



Primitive Forms. Archi-Ch^topoda or 

 Archi-Annelida 



There are a few small, simple marine worms, with some Annelid or 

 Chaetopod characters, which are sometimes supposed to be ancestral 

 forms. Thus DinophiUis is a minute Planarian-like animal found 

 among Algae. In the young at least the body is distinctly segmented, 

 but there are no bristles, gills, or tentacles. There are circling band* 

 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- 

 drilus, Saccocirrus, and Histriodrilus. 



The small body is segmented and uniform ; there are no setae, 

 parapodia, cutI, 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. 143 (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 ciUa 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 larvae are typical 

 Trojchospheres, ciliated, free-swimming, light -loving, surface animals, 

 feeding on minute pelagic organisms, seeking the depths as age advances. 



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. Histriodrilus is parasitic on the eggs of the 

 lobster, and its affinities are doubtful. 



Appendix (2) to Chcetopoda 



Parasitic and Degenerate Chaetopods. Myzostomata 



The remarkable forms {Myzostorna) included in this small class, live 

 parasitically on feather-stars, on which they form galls. They are 



