206. CHAPTERS ON EVOLUTION. 



completely, and appear as sausage-like, sack-shaped, or discoidal 

 excrescences of their host, filled with ova (or eggs) ; from the point of 

 attachment closed tubes, ramified like roots (Fig. 118), sink into the 

 interior of the host, twisting round its intestine, or becoming diffused 

 among the sac-like tubes of its liver. The only manifestations of life 

 which persist in these non plus ultras in the series of retrogressively 

 metamorphosed Crustacea, are powerful contractions of the roots, 

 and an alternate expansion and contraction of the body, in conse- 

 quence of which water flows into the brood-cavity and is again 

 expelled through a wide orifice." 



Now, the history of Sacculina-development clearly proves its 

 relationship with other crustaceans. As an adult, a Sacculina might 

 literally be anything in the way of animal organisation. It is a bag 

 filled with eggs, and attached by roots to a hermit-crab. As such, 

 its true nature is not recognisable by any of the deductions to be 

 drawn from the ordinary facts of animal structure. Development, 

 however, not only shows us its descent, but settles its place in 

 the animal scale by declaring its affinities, not only with the 

 Barnacles, but with other crustaceans. From each egg contained 

 within the bag-like body, there is developed a little free-swimming 

 creature (Fig. 121). This embryo possesses an oval body, ending 

 in two short processes; three pairs of swimming 

 feet are developed ; a single eye may or may not 

 be present ; but we find in the young Sacculina 

 a clear and unmistakable reproduction of the 

 "Nauplius" (Fig. 119) of a Barnacle. No mouth 

 or digestive system, however, exists in the 

 youthful Sacculina, which shortly changes into 

 the " pupa " state (Fig. 120, c). Here it closely 

 resembles the Cypris water-flea (Fig. 116, B), 

 whose development we shall also presently note. It 

 possesses a shell folded down at the edges so as to 

 enclose the body. The front pair of limbs, as in the 

 Barnacle, become modified to form organs of attachment ; the two 

 remaining pairs of feet are cast off; and, as in the Barnacle, six pairs of 

 forked swimming feet appear on the body behind, while the forked 

 tail is also a characteristic feature of the young Sacculina. Then 

 succeeds the stage of attachment. The front feet, or feelers, serve as 

 means of fixation to the body of the crab-host. The remaining six 

 pairs of feet are cast off ; the roots are developed from the feelers, and 

 the animal thus assumes the adult sac-like and degraded form. Thus a 

 Sacculina and its parasitic neighbours closely resemble barnacles up 

 to the pupa-stage. At this point the evolution manifested in 

 " degradation " of the Sacculina intervenes, and the six pairs of feet, 

 which in the Barnacles are converted into the " cirri " or " plumes," 



