IX.] RETROGRESSIVE DEVELOPMENT IN NATURE. II 



This is, however, but the first step in retrogressive develop- 

 ment : more marked effects are witnessed in forms which are 

 more completely and permanently fixed to their hosts. To 

 the same crustacean order belong the Enioniscidae, which are 

 internally parasitic upon other crustaceans, especially upon the 

 common shore-crab (Carcinus maenas). During their whole 

 life, these parasites never leave the host, nor move from the 

 position they have once taken up within it. They live attached 

 to its liver, sucking the juices ; after growing enormously and 

 producing thousands and thousands of eggs, they finally die. It 

 is clear that such a mode of life must render superfluous, and 

 therefore degenerate, many structures which were essential to 

 their free-swimming ancestors. This retrogression takes place 

 to such a degree, and the whole structure of the animal is thereby 

 so modified and altered that they are scarcely recognizable 

 as Crustacea. The characteristic segmentation of the body 

 is entirely lost, and the hard exo-skeleton is replaced by a thin 

 soft skin. The body lengthens to a vermiform shape, acquires 

 peculiar pointed appendages for the reception of the eggs, and 

 becomes colourless, like that of all animals which live in the 

 dark. All these modifications are quite intelligible ; the seg- 

 mentation of the crustacean body facilitates movement, while the 

 hard exo-skeleton serves for the attachment of muscles. The 

 eyes and antennae completely disappear, because the animal 

 lives in darkness, and does not need to see, and because the 

 sense of touch is unnecessary to it after it has once taken up 

 its position. Not a vestige remains of certain mouth-organs 

 which are well developed in allied species ; and the legs, 

 of which free-swimming forms have seven thoracic and six 

 abdominal pairs, are reduced in number. The internal organs 

 are also reduced, with the single exception of the ovaries, 

 which increase so much in size that the animal appears hke 

 a mere bag of eggs. 



It may now be asked how we know this peculiar vermiform 

 being to be a crustacean and an Isopod at all. We know this 

 to be a fact because there are many other parasitic Isopods in 

 which degeneration has not gone so far, and which present 

 well-marked stages of transition from the above-mentioned 

 fish-lice to the Enioniscidae. Furthermore, the descent of the 

 Entoniscidae from free-swimming forms is clearly proved by 



