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for a time it advances, yet after a particular period has 
arrived the remainder of its development is retrogressive. 
The various appendages in each parasite are developed in 
the same order. In the one they become perfected when 
the creature is fully developed. In the other, long before 
the animal has reached maturity some have disappeared, 
the remainder continue in a rudimentary condition, and 
it is incapable of further movement. The internal organs 
of both copepods are developed in the same way. In one 
they continue advancing until perfected, and the animal 
is thus capable of living for considerable periods apart 
from its host. In the other, such organs as the digestive 
gland, the brain and nerves, and the blood system become 
rudimentary, if they do not altogether disappear. The 
ovary loses its original position and passes into the genital 
segment. The animal dies when removed from its host. 
If only the adults were known, it would practically be 
impossible to recognise that such a form as Lern@a was im 
any way related to such a typical free-swimming Copepod 
as Calanus, and it would therefore still occupy 
an uncertain position. But when the whole life 
history of both copepods is known, tracing the connection 
becomes comparatively easy. Both originate from a free 
larval stage known as the nauplius, which has been 
regarded as the representative of a far back common 
ancestor. Both pass through a cyclops stage. The 
one ancestral cyclops form, we may suppose, by maintain- 
ing a free swimming life, gradually acquired more perfect 
appendages, and became at last the form now known as 
Calanus. The other cyclops form by adopting a sedentary 
life, and depending on other animals for its food, became 
semi-parasitic like many of the ascidian- and sponge-fre- 
quenting forms of copepoda. ‘The transition from 
Lichomolgus-like copepods to such forms as Bomolochus 
