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Updated: June 26, 2025
How later Crustacea increase the toughness of the coat by deposits of lime, and lead on to the crab and lobster, and how one early branch invades the land, develops air-breathing apparatus, and culminates in the spiders and insects, will be considered later. We shall see that there is most remarkable evidence connecting the highest of the Arthropods, the insect, with a remote Annelid ancestor.
But the ancestors of insects looked more like thousand-legs or centipedes, and here head and thorax are much less distinct. But in the annelid the mouth is on the second segment; here it is on the fourth. It has evidently travelled backward.
Thus, for instance, an insect, standing at the head of the articulated animals, is, in the larva state, a true annelid, or worm, the annelida being the lowest in the same class. The embryo of a crab resembles the perfect animal of the inferior order myriapoda, and passes through all the forms of transition which characterize the intermediate tribes of crustacea.
His name is Pectinaria Belgica. He is an Annelid, or true worm, connected with the Serpulea and Sabellae of which I have spoken already, and holds himself in his case like them, by hooks and bristles set on each ring of his body. But if you have courage to run out fifteen or twenty miles to the Diamond, you may find really rare and valuable animals.
While grazing these rocky slopes lost under the waters, I still spotted some seashells, tube worms, lively annelid worms from the genus Spirorbis, and certain starfish specimens.
The work of locomotion has been transferred from the trunk to the legs. The abdomen of the insect is as clearly composed of distinct segments as the body of the annelid. Of these there are perhaps typically eleven. The thorax is composed of three segments, distinct in the lowest forms, fused in the highest.
Here a graceful Annelid describes Hogarth's line of beauty upon the sandy bottom. There another glides over the surface with sinuous course, rowed by more oars than a Venetian galley, more brilliant in its iridescence than the barge of Cleopatra, albeit "The poop was beaten gold, Purple the sails."
The annelid had two anterior segments differing from those of the rest of the body; these may, perhaps, be considered as the foreshadowings of a structure not yet realized; they can only by courtesy be called a head. Thirdly, the insect has legs. The annelid had fin-like parapodia, approaching the legs of insects about as closely as the fins of a fish approach the legs of a mammal.
With fishes, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz Muller, has as sole companion and competitor on the barren sandy shore of South Brazil, an anomalous annelid.
To begin with less characteristic organs the digestive system is much like that of the annelid or schematic worm, but with greatly increased glandular and absorptive surfaces. The present mouth of nearly all vertebrates is probably not primitive. It is almost certainly one of the gill-slits of some old ancestor of fish, such as now are used to discharge the water which is used for respiration.
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