He did not confine himself in his studies, however, to the physiology and pathology, nor even to the anatomy and embryology of man. After 1840 he devoted himself to the study of invertebrates and investigated the starfish and the pentacrinites. While engaged in his work on the invertebrates he found that the fossil remains of animals had not been carefully explored, so for a time he devoted himself to paleontology. While his salary as professor was ample for his own support, it was not what would be called generous at the present time, yet Müller became so devoted to his science that he paid certain of the workmen to be on the lookout for fossil remains for him in the quarries of the Eifel. He became deeply interested, too, in life in the sea and made his vacations times of specially hard work, investigating the conditions of low life among marine organisms. He passed from one class of life to another. From sea-urchins and starfish to infusoria and polycystina, whose varieties he was himself the first to recognize and describe. Müller was one of the first to point out that certain of the lower animals could propagate similar and dissimilar {238} generations, that is, reproduce by alternate generations. He studied and demonstrated especially the metamorphoses in the echinodermata, and his broad vision and careful observation in this new and surprising scientific field cleared up many things that had been mysteries before. In paleontology Müller worked with our own Agassiz, then a young man, or perhaps it should rather be said that Agassiz worked with Müller. A paper, for whose compilation they made a series of observations together, appeared at Neufchatel, in 1834. It was a note on the vertebrae of living and fossil dog fishes. At this time Müller was interested in fossil fishes of many kinds and wrote several articles in later years on this subject. Toward the end of Müller's life he studied especially the polycystina, certain of the radiolaria, and some of the many chambered specimens, fossil and living, that were attracting much attention at that time. As a matter of fact he went the day before his death to the zoological museum of Professor Peters in Berlin, in order to obtain some polythalamacea. How open to advance in science and how ready to encourage the work of others Müller was, may be gathered from his attitude to parasites as the cause of disease, when these began to be discovered. After Professor Schoenlien's discovery of the parasite of favus, Müller became interested in it, confirmed Schoenlein's observations and added something to our knowledge of it. About this time, also, he discovered the psorosperm as a parasite of animals and possibly of man, and devoted considerable attention to it. His work was afterward greatly extended by one of his pupils, Lieberkühn, whose researches with regard to these minute organisms attracted the attention of the medical world.