234 PHYL UM ANNELIDA. 



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

 differs markedly in the different genera. In Echitirus 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 Chatopoda 



Primitive Forms. ARCHI-CH^ETOPODA 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 Dinophilus 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 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- 

 drilus, SaccocirruS) and Histriodrilus. 



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

 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. 1 19 (i i)) 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 larvae 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. 



Prolodrilus 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. 



