This state of affairs has no antecedent improbability about it, since in the Vertebrata the coelom is unquestionably confluent with the haemal system through the lymphatic vessels.
One result of this among the Vertebrata is that the eyeball is pink in colour, since the cornea, iris and retina being transparent, the red blood contained in the capillaries is unmasked by the absence of pigmentary material.
It is now applied throughout the Vertebrata of both Mesozoic and Cenozoic times.
The most that can be said is that the Chaetognaths begin life with three segments, a feature they share with such widelydiffering groups as the Brachiopoda, the Echinoderma and the Enteropneusta, and probably Vertebrata generally.
Sir Richard Owen, in his work on The Anatomy of Vertebrates, followed Latreille in dividing the Vertebrata into Haematotherma and Haematocrya, and adopted Leuckart's term of Dipnoa for the Amphibia.
This division of the Vertebrata into hot and cold blooded is a curiously retrograde step, only intelligible when we reflect that the excellent entomologist had no real comprehension of vertebrate morphology; but he makes some atonement for the blunder by steadily upholding the class distinctness of the Amphibia.
In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley divided the Vertebrata into Mammals, Sauroids and Ichthyoids, the latter division containing the Amphibia and Pisces.