132 LECTURES ON BIOLOGY 



star-fishes, holothurians, sea-urchins, ophiuroids, and finally in 

 the Balanoglossus, are found in other vermes and also in many 

 molluscs. Interesting conditions are observed in the annelids. 

 Among the marine forms of this class we find a free-swimming 

 pelagic larval form, the celebrated Trochophora. By gradual 

 metamorphoses, growing longitudinally, this larva is trans- 

 formed into an adult worm. In distinction to the pelagic annelids 

 the freshwater forms have no intermediate larval stage ; the ovum 

 produces the fully developed worm. Nevertheless, there is no 

 doubt that the freshwater annelids formerly passed through a 

 larval stage, for in the growing embryo we find indubitable 

 characteristics of Trochophora. 



Finally let us consider an instance taken from the crustaceans. 

 Who has not heard of the duck-mussel (Lepas anatifera), which 

 in the old German legend was the young 

 stage of the barnacle (or barnicle) goose. 

 According to old ideas, the distinctive feature 

 between the animal and vegetable kingdom 

 was the ability of locomotion. Hence, corals 

 and sea-anemones, sponges and tunicates 

 were even until recent times counted among 

 the flora. Duck-mussels, being fixed, were 

 regarded in mythical zoology as plants, and 

 A MARINE ANNELID. the monks in mediaeval times were not slow 

 in arguing that as duck-mussels were plants, 

 so the geese and ducks that developed from them must also 

 be 'plants,' and were not therefore included in the rule which 

 forbade on certain days the eating of flesh. 



The position in the system of zoology of the Cirripedes, of 

 which I have mentioned the duck-mussel as the best known type, 

 was long obscure. Almost universally these animals were regarded 

 as mussels. The calcareous plates which encase their body and 

 greatly resemble mussel-shells not unnaturally gave rise to this 

 error. But now we know that the Cirripedes are crustaceans, 

 though when we fish a piece of driftwood or pumice-stone from 

 the sea and observe on the underside ' mussels ' anchored by a 

 strong leather- stalk, and projecting from the partly open valves 

 numerous cirral threads, we see a picture widely differing from 

 our usual conception of crustaceans. But a knowledge of the 

 phylogeny of Cirripedes leaves no room for doubt. 



