From T. H. Huxley [9–12 March 1859]1
1) Serial homologies are without doubt far less readily traceable in the Mollusca than in the Annulosa just because their body is never divided into true segments—corresponding with those observable in the Vertebrata & Annulosa 2 However I should say that the pedal ganglia are the serial homologues of the cerebral ganglia—
The arms of the Cephalopoda afford another case which is much more genuine than it looks inasmuch as their original paired successive arrangement on each side of the foot is disguised by subsequent modifications The chambers of the Nautilus shell are serial homologues—so are the paired branchiæ of the Eolidæ — so are the multitudinous eyes of Pecten —; so are (perhaps) the palps and gills of Lamellibranchiata —indeed, on second thoughts, I would knock out the perhaps.
The valves of the shell of Chiton afford another case—and if one were very hard up perhaps the palettes of Teredo might be said to be the serial homologues of the proper shell valves—
2. The multiplication of suckers on the Dibranchiate Cephalopod arm—the multiplication of the tentacles in the Nautilus.: the multiplication of branchiæ in the Nudibranchs.; of Eyes in the Pectens might all be brought forward as cases of irrelative or vegetative (which is as you say a bad word) repetition—but there are very few good instances of anything of the sort among Mollusks that I can recollect
3. The Echini would afford capital examples of the indefinite repetition of homological whorls of homological parts— For the shell, consists of a great number of superimposed circles each of which consists of five sets of homologous parts each set containing those homologous pieces twice or more repeated.
As to the Encrinites the stem is a series of homologous joints—the calyx is a series of whorls of homologous parts & the arms are series of homologous joints but I do not know that you can say that the elements of the stem are homologous with those of the calyx or with those of the arms
The rays of Asteriadæ can hardly be called unimportant parts as they contain portions of the all the organs of the body— In the Medusidæ again the number of reproductive organs in a whorl is often very great— But the strongest cases against your view are afforded by the Calycophoridæ (Diphydæ) and Physophoridæ here you may have an organism a yard long with regularly repeated homologous organs in great number—thus [DIAGRAM HERE] generative organs Gen. org leaf like appendages Float stomachs & tentacles St. & T propelling organs
Now these organs have just the same primitive relation to one another as the modified leaves which compose the different whorls of a flower. They are not in whorls however but are on one side of a stem—3
To tell you the truth I learnt the meaning of ‘homology’ from studying these creatures and my hair stands on end at even the hint that there are no serial homologies among them—4
What do you say to a Tænia too? He is a Cuvierian Radiate
Again the segments of Nodosaria or other chambered Foraminifer are serial homologues n’est ce pas?
How about the successive pinnæ of Pennatula
Serial homologies in the Mollusca. Gives instances of repetition of homological parts in Radiata.
Please cite as
Darwin Correspondence Project, “Letter no. 2427,” accessed on 3 December 2016, http://www.darwinproject.ac.uk/DCP-LETT-2427