SEA-FISHERIES LABORATORY. 233 



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 Lemma was in 

 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 Bomolockm 



