334 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Analogy. This term is applied to chemical units playing the same role 

 in a biochemical system. Phosphocreatine and phosphoarginine in the 

 muscles of mammals and crustaceans respectively, are analogues. Bio- 

 chemical units may be both isologues and analogues, as is the case for a 

 blood haemoglobin and a blood chlorocruorin : both serve as oxygen 

 carriers. But this is not necessarily so : a blood haemoglobin and a 

 haemocyanin are analogues, but they are not isologues. 



When we have information about the relative positions in the classifica- 

 tion of a number of species, and about the isologic relations of one or other 

 of their biochemical constituents, then it becomes possible to draw con- 

 clusions about the biochemical evolution of these constituents. Consider, 

 for example, chlorocruorin, the oxygen carrier in the Chlorhemians, in the 

 Sabellidae and the Serpulidae. This carrier is present in the blood of 

 three families of polychete Annelids. The Chlorhemians are Spionids, a 

 classification which groups among the sedentary polychete Annelids, which 

 are descended from the errant polychete Annelids, those forms whose 

 preoral lobe is not sunk into the first segment of the metasome, and which 

 feed on floating plankton gathered by means of posterior antennae in the 

 form of long palps bearing a ciliated gutter. They live in sand or mud and 

 secrete a membranous tube covered with a fine layer of slime. The 

 Chlorhemians are Spionids which have lost the dissepiments and even the 

 external segmentation. Their blood is green, and their palps are folded 

 forward. Related to the Spionids are the cryptocephalic Annelids, having 

 a preoral lobe sunk into the first segment of the trunk, but possessing 

 furrowed appendages like those of the Spionids. Sedentary and tubicolous, 

 the Cryptocephalae comprise two sub-divisions, the Sabellariides, which 

 although sedentary and microphagic, have retained an uneven number 

 of antennae, and the Sabelliforms which have even antennae and palps 

 forming a multicoloured corolla. The Sabelliforms are divided into the 

 Sabellidae, having a mucous tube, membranous or cornified, and the 

 Serpulidae, possessing a calcareous tube. 



The blood of Spionids other than the Chlorhemians is coloured red by 

 haemoglobin. When we come to the Sabellariides, one of the genera of 

 which is Sabellaria, their blood is charged with haemoglobin. In the 

 Sabelliforms, sometimes called the Serpuliforms, chlorocruorin is the 

 characteristic blood pigment. All the Sabellides so far studied contain it. 

 Among the Serpulides blood of species in the genus Serpula contain 

 both chlorocruorin and haemoglobin and in the genus Spirorbis, one 

 species, S. borealis has a blood coloured by chlorocruorin, another, 

 S. corrugatus contains haemoglobin and a third, S. militaris has a colourless 

 blood. H. M. Fox (1949) did not find chlorocruorin in the tissues or in the 

 coelomic fluid of the forms having chlorocruorin in the blood. No doubt, 

 in those forms which contain it, the synthesis of chlorocruorin is a variant of 



