206 SEGMENTED WORMS OR ANNELIDA. 



parapodia, cirri, and gills are absent, but except in the degenerate 

 males a few setae are always present. The most characteristic structure 

 is the elongated solid proboscis, which has the mouth at its base. The 

 nervous system consists of a gullet-ring and a ventral cord, but the 

 latter is unsegmented, and there is no brain. The gut is coiled, and 

 bears a curious adjacent tube known as the "collateral intestine," and 

 a. pair of excretory "anal vesicles," opening from gut to body cavity, 

 and formed in development from nephridia. The anus is terminal, 

 there is a closed vascular system, and one to three pairs of nephridia. 

 The sexes are separate, the reproductive elements are formed on the 

 walls of the body cavity, and are shed into it. 



There is a metamorphosis in development, but the nature of the larva 

 differs markedly in the different genera. In Echiurus 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 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 

 Chsetopod characters, which are sometimes supposed to be ancestral 

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

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

 but there are no bristles, gills, or tentacles. The nervous system 

 consists of a brain and a ventral ganglionated cord, but it remains 

 embedded in the epidermis. 



More distinctly Annelid are the marine worms Polygordius, 

 Protodrilus, and Histriodrilus. 



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

 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. 89 (n)) 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 

 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 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, 



