SEX DIMORPHISM 



269 



B.L. 



PR. 



tubes ; the common Terebella or Lanice conchilega, with its tubes of 

 glued sand particles ; and the strange phosphorescent Chceiopterus, 

 found in deep water, within its yellow parchment-like tube. 



III. Echiuridae.— In holes in the rocks on the warmer coasts of 

 Europe there lives a curious " worm " — 

 Bonellia viridis, of a beautiful green colour, 

 with a globular body, and a long, grooved, 

 anteriorly forked, pre-oral protrusion. 

 Such, at least, is the female ; but the 

 male is microscopic in size, lives in or on 

 his mate, and is exceedingly degenerate. 

 His gut is without mouth and anus, the 

 surface is covered with cilia, and the body 

 cavity almost obliterated. Related to 

 Bonellia, but of less anomalous shape, are 

 a few other forms, like Thalassema and 

 Echiurus. 



In all, the body in the adult shows mere 

 traces of segmentation ; parapodia, cirri, 

 a'Vid gills are absent, but, except in the 

 degenerate males, a few setae are always 

 present. The most characteristic structure 

 is the elongated sohd 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 " col- 

 lateral intestine," and a pair of excretory . 

 " anal vesicles," opening from gut to body 

 cavity, and formed in development from MALE 

 nephridia. The anus is terminal, there is 

 a closed vascular system, and one to four 

 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 develop- 

 ment, but the natiire of the larva differs 

 markedly in the different genera. In 



Mo. 



..io 



Lo'oj 



F 



Fig. 146. — Sex dimorphism in Bonellia 

 viridis, about natural size. 



The female (F.) has a body about 2 inches long, 

 bearing a pre-oral, ventrally-grooved proboscis 

 {PR.), often 6 inches or more in length when fully 

 extended. The proboscis ends in two arms (B.L.), 

 often recurved. The ciliated groove of the food- 

 catching proboscis leads into the mouth (Mo.). 

 The degenerate male (M.) is about a sixteenth 

 of an inch in length and lives as a parasite, first 

 on'the proboscis, then in the mouth, and finally 

 in^the nephridium of the female. 



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