CLASSIFICATION OF CHALTOPODA. 211 



never " jaws." Most of them feed on minute Algse swept in by the cilia 

 on the tentacles and other structures about the mouth. 



The fisherman's lob-worm (Arenicola) burrows on the sandy shore 

 like Lumbricus in the fields. Common also on the shore within a tube 

 of glued sand particles is Terebella or Lanice conchilega^ where the ex- 

 cretory tubes are partly united by a longitudinal tube in a manner sug- 

 gestive of the segmental duct which connects the nephridia of a young 

 Vertebrate. The twisted limy tubes of Serpula are common outside 

 shells and all sorts of marine objects, and the animal bears a stopper 

 or operculum, with which it closes the mouth of its tube, but through 

 which it probably at the same time breathes. In deep water, within 

 a yellow parchment -like tube, Chatopterus may be dredged, perhaps 

 the strangest form of all. 



III. Echiuridce. 



In holes in the rocks on some of the warmer European coasts lives a 

 curious "worm" Bonellia viridis, of a beautiful green colour, with a 

 globular body and a long, grooved, anteriorly forked, pre-oral protru- 

 sion. Such, at least, is the female, but the male is microscopic in size, 

 hopelessly degenerate, living parasitically in or on his mate. The 

 male resembles in some ways a Turbellarian, is mouthless and gutless, 

 and little else than a migratory spermatophore. By means of cilia, it 

 moves from one part of the female to another, and fertilises the eggs in 

 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- 

 assenia. 



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 setae are always represented (except in the male 

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

 as well. The nerve cord is unsegmented, 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 larvse differing from the 

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



Appendix (i) to Chcetopoda. 



PRIMITIVE CHyETOPODS AND ANNELIDS (Archi-Choetopoda and 

 Archi-Annelida). 



An aberrant Chsetopod type is represented by Saccocirrus, a small 

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



