THE ARGUMENT FROM EMBRYOLOGY S3 



Fig. 11. 



Young and adult specimens of Sacculina, to illustrate the Degeneration or Retrograde 

 Metamorphosis which the parasite undergoes in the course of its development. 



A. — The Nauplius stage, in which the young Sacculina hatches. The three pairs 

 of appendages correspond to the antennules, antennas, and mandibles of a Crab or 

 Crayfish. The black spot between the antennules is the eye, and the small patch 

 immediately behind the eye and between the two hinder pairs of appendages is the 

 ovary, which is already present at this very early stage. Unlike the Nauplius stage 

 of other groups of Crustacea the Sacculina Nauplius has no mouth or alimentary 

 canal. x go. 



B. — The Cypris or pupa stage. It is at this stage that the young Sacculma, hitherto 

 a free-swimminer animal, attaches itself to a Crab and becomes parasitic. The pupa 

 is characterised by the bivalved carapace ; the stout antennules by which it fixes 

 itself to the Crab ; and the six pairs of locomotor appendages. The black spot is the 

 Nauplius eye, and the mass immediately below it is the ovary. xqo. 



C. — The Sacculina three days after fixing itself to the Crab. The six pairs of 

 swimming legs have rotted away and fallen off; the bivalved carapace is being 

 detached, and is carrying with it the eye and certain debris from the body. The 

 sole parts remaining, out of which the adult Sacculina will be formed, are the 

 antennules, now modified into a tube, which is represented projecting through a 

 piece of the skin of the Crab ; and the head, which forms a bottle-shaped mass 

 attached to the tube, and containing the ovary. x 90. 



D. — Adult Sacculina attached to the ventral surface of the tail of a Crab (Por- 

 tunus). The Sacculina is the large dark-coloured bag in the lower part of the 

 figure; it is attached to the Crab by a short fleshy stalk, not seen in the figure, 

 which, penetrating the skin spreads out into a tuft of branching roots in the 

 •Crabs body. x£ 



