CLASSIFICATION OF CHMTOPODA. 187 



a modified excretory tube, which serves the female Bonellia as a uterus. 

 Here illustrated in extreme, we see the usual inequality (in size) between 

 the sexes. 



Less abnormal than Bonellia, are the genera Echiurus and Thal- 

 assema. 



In this small sub-order, the adults have at most indistinct traces of the 

 segments which the young forms exhibit. Nor are there parapodia, 

 cirri, or gills, but sets are always represented (except in the male 

 Bonellia) by two anterior bristles, and in Echiurus by posterior spines 

 as well. The nerve-cord is imsegmented, and there is but a slight 

 anterior ring without a brain. The anterior part of the body forms a 

 muscular, well-innervated, ciliated proboscis, with the mouth deeply 

 situated at its base ; the gut is much coiled, bears a curious adjacent tube 

 known as the "collateral intestine," and a pair of excretory "anal 

 glands " opening into the body-cavity by ciliated funnels. There is a 

 terminal anus. There are dorsal and ventral blood-vessels, and two or 

 three pairs of nephridia, one or more of which function as reproductive 

 ducts. The sexes are separate, and the reproductive elements are 

 formed on the walls of the body cavity, into which they are liberated. 

 There is a metamorphosis in development, the larvae differing from the 

 adults in many ways, e.g., in being segmented. 



Appendix {l) to Chatopoda. 



Primitive Ch.«topods and Annelids (Archi-ChEetopoda and 

 Archi-Annelida). 



An aberrant Chsetopod type is represented by Saccocirrtis, a small 

 marine "worm" with many primitive characteristics. The body is 

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

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

 the nervous system remains embedded in the epidermis. 



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

 Polygordius, Protodrilus, and Histriodrilus — all marine. The small 

 body is segmented and uniform ; there are no setae, 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. 



Polygordius 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 larvK seem to throw some light on 

 the nature of the ancestral Annelids. They are cihated, free-swimming, 

 light-loving, surface animals, feeding on minute pelagic animals, seeking 

 the depths as age advances. According to some, the larva represents a 

 primitive unsegmented ancestral Annelid with medusoid affinities ; 



