CHAPTER X 

 THE EYES OF CYCLOSTOMES 



Although he made a classical description of the eyes of all classes of 

 Vertebrates except Cyclostomes, I am introducing this chapter which is the 

 first of a series dealing with the structure of the eyes of Vertebrates with the 

 portrait of detmar wilhelm soemmerring (1793-1871) (Fig. 269) in view of 

 the fact that he was one of the earliest writers to make a systematic study of this 

 subject. It is true that many incidental observations had been made on the 

 fijier structure of the eyes of different Vertebrates by such investigators as van 

 Leeuwenhoek,^ Zinn ^ and Young,^ while compendia had been published by such 

 authors as Bluraenbach,* Albers,^ and Cuvier ® ; but none is so delightful to 

 read as is the thesis written in Latin which brought Soemmerring his doctorate 

 in Gottingen in 1816, and was published in 1818 under the title De oculorum 

 hominis animaliumque sectione horizontali commentatio ; the illustrations are so 

 beautiful that several of them have been reproduced in the following chapters. 

 D. W. Soemmerring, the son of an equally distinguished German ophtha,lmologist, 

 S. T. von Soemmerring (who, it will be remembered, first described the macula 

 lutea as a hole in the retina), was born in Frankfurt where in later life he practised 

 for many years and where his jubilee as a doctor was officially celebrated in 

 1866. He is also remembered ophthalmologically for two particular observations 

 — a description of the organic changes in the eye after the operation for cataract 

 in which he described the annular remnant of the lens now universally known 

 as Soemmerring's ring (1828), and the first description of a living cysticercus in 

 the human eye (1830). 



The CYCLOSTOMES (kJk/\o9. round ; otoixx, a mouth), so called 

 because of their round, jawless, suctorial mouths which differentiate 

 them from all other Vertebrates, are the only surviving representatives 

 of the large class of agnatha (a, privative ; yvddos, jaw) which flourished 

 in great variety and numbers during Palaeozoic times and are now 

 with this exception extinct. They are freely-swimming worm -like 

 " pre-fishes "' of extreme antiquity, essentially primitive in their 

 structure and differing in many ways from true Fishes, principally in 

 the absence of jaws, by the single olfactory organ and by the absence 

 of paired fins. Today they are represented by two existing types and 

 a few others like them — the hagfishes (slime-eels) and the lampreys. 

 The eyes of the former, buried deeply within the skm, are degenerate 

 and sightless and are described at a later stage^ ; those of the latter, 

 at first buried and later coming to the surface, constitute the most 



' Epistolce physiologicce, Delphis, 1719. 

 - Comment. Soc. Sci., Gottingen, 1754. 

 3 Philos. Trans., 1793. 



* Vergl. Anat., 1784. 



5 Beyt. z. Anat. u. Physiol, d. Thiere, 1802. 



* Leg-ons d'anat. comparee, Paris, 1805. 

 ' p. 734. 



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