348 " THE INFLUENCE OF LIVING SURROUNDINGS. 



body, since these waste products are in the highest degree pre- 

 judicial to life. Parasites permanently attached to their host 

 and living on its juices have no need of most of these organs, 

 and, in fact, in all such parasites all or most of them have 

 totally disappeared or are extremely degenerate; the degree of 

 degeneration is, however, certainly very different in different 

 species of parasites. 



With reference to this it will suffice to select a few of the 

 best-known and more instructive examples from the abundance 

 at our disposal. Among inollusca there is first and foremost the 



FIG. 94. Entoconcha mirabilit, Jlilller. <5, whcu sexually mature, in the form of a spiral 

 worm-like creature in the body cavity of Synapta diyitata. , the larva of the 

 mollusc. 



well-known Entoconclia (see fig. 94) ; this consists of a simple 

 sac containing nothing but the hermaphrodite organs and 

 the embryos of a univalve mollusc. These embiyos have 

 precisely the form and structure of the ordinary larvae of 

 univalves adapted to a free existence ; an oval shell with an 

 operculum to fit the mouth, an organ for swimming known as 

 the velum, siich as occurs in many similar larvfe a brain and 

 auditory organ, intestines, gill cavity and all the other parts. 

 But all the organs here enumerated are entirely lost when the 



