The fins of Pteropods are now interpreted as the expanded lateral margins of the foot, termed parapodia, not homologous with the siphonof Cephalopods which is formed from epipodia.
An important nucleo-proteid is haemoglobulin or haemoglobin, the colouring matter of the red blood corpuscles of vertebrates; a related substance, haemocyanin, in which the iron of haemoglobin is replaced by copper, occurs in the blood of cephalopods and crayfish.
Kolliker (Development of Cephalopods, 1844), Remak (Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kovalevsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier.
The metacestode-larva occurs free in the intestine of fish, Cephalopods and crabs, and is known as Scolex polymorphus.
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