CELETOPODA 307 



complicated history, which cannot be satisfactorily explained 

 on any other view. 



Gephykea. These are worm-like marine animals which 

 burrow in sand or mud, or live in holes and crevices in rocks. 

 Though the number of species is small, their organisation is so 

 peculiar that they cannot well be included in any of the 

 larger sub -kingdoms, and must therefore be considered as 

 forming a sub-kingdom by themselves. They are unsegmented 

 animals, without lateral appendages, and the skin is soft and 

 naked. Some of them possess bristles which resemble those 

 of the Chsetopoda. Sexual dimorphism does not appear to be 

 exhibited by any of the Gephyrea except one, namely, Bonellia, 

 but in this genus it is carried to an extreme degree. 



Bonellia is a greenish-looking, sausage-shaped creature, at 

 the anterior end of which is a long, flexible, fleshy tentacle, 

 divided into two branches at the extremity. The body is an 

 inch or two in length, the tentacle longer. This description 

 applies only to the female. The male is only about one- 

 hundredth the size, and lives parasitically, actually within 

 the oviduct of the female. In the small male the tentacle 

 is wanting. 



The degenerate condition of the male in this case as in 

 others, and as in both sexes of many parasitic animals, 

 may be attributed to the direct effect of the parasitic mode 

 of life. On the principle which I have enunciated in this 

 work of the heredity of acquired characters in association 

 with the conditions under which they were acquired, we can 

 understand why the effects of the parasitic habit have been 

 transmitted only to the males. The development of both 

 sexes of Bonellia has been investigated. At a very early 

 stage after hatching the male embryos attach themselves to 

 the tentacle of an adult female. Up to this time they move 

 by means of the minute vibrating processes known as cilia, 

 but these are now lost. No mouth or anus is formed, but the 



