212 SEGMENTED WORMS OR ANNELIDA. 



segmented, and very uniform throughout ; the pre-oral region is small, 

 but the mouth segment is large ; there are bundles of setfe on the rings ; 

 the nervous system remains embedded in the epidermis. 



More primitive, however, are the Archi-Annelida represented by 

 Polygordiiis, Protodrihis., and Hisiriodrilus — all marine. The small 

 body is segmented and uniform ; there are no setK, parapodia, cirri, 

 or gills, but the head bears a few tentacles ; as in Saccocirrus, the 

 pre-oral region is small, and the segment around the mouth is large ; 

 the very simple nervous system is retained in the epidermis. 



Pofygorduis 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 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 uni- 

 cellular 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 seem to throw some light on the nature 

 of the ancestral Annelids. They 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 Turl:)ellarian-like, the reproductive organs hermaphrodite, 

 the development direct. Histriodribis is parasitic on the eggs of the 

 lobster. 



Appendix {2) to CluTtopoda. 

 Parasitic and Degenerate Ch/etopods. Mvzostomata. 



The remarkable forms (Myzostonia) included in this small class, live 

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

 regarded as divergent offshoots from primitive Annelids, the larval form 

 showing some distinctly Cha^topod characters. The minute disc-like 

 body is unsegmented, and Ijears five pairs of parapodia, each with a 

 grappling hook, with which five pairs of suckers usually alternate. 

 There are also abundant cirri. The skin is thick, the body muscular, 

 the nervous system is concentrated in a ganglionic mass,- which encircles 

 the gullet, and gives oft' afiundant branches. There is a protrusible 

 proboscis and a branched gut ; the mouth and anus are ventral. The 

 ova arise in the reduced body cavity, and pass by tlrree meandering 

 oviducts to the anal aperture. The testes are paired, branched, and 

 ventral, with associated ducts, which open anteriorly on the side of the 

 body. The sexual relations are interesting, for one species is herma- 

 phrodite and another unisexual, between which there is an intermediate 

 species with ovaries and rudimentary testes. The hermaphrodite form 

 may bear on its body dwarfish males, analogous to the complemental 

 pigmies on some hermaphrodite barnacles. 



