490 



W. K. Spencer — Hypostomic Eyes of Trilohites. 



median and unpaired in BrancJiipus, yet an exactly similar organ 

 described by Patten ^ in bis account of the development of Limulus 

 possesses a paired origin. The paired ' Anlagen/ however, 

 afterwards fuse and take up a median position. There is no doubt 

 that if the development of the ' Kolbenzellen ' organ of JSranchipus 

 were followed it would also have a paired origin. 



Fig. 1. 



Fig. 3. 



Fig. 2. 



Fig. 1. — Hypostoma of Bronteus, After Lindstrom. m = macula. 



Fig. 2. — Vertical section of macula of Bronteus polyactin. Siity times nat. size. 



After Lindstrom. a — lenses. 

 Fig. 3. — Group of cells from the ' Kolbenzellen ' organ, showing their ommatidia- 



like arrangement, r = rhabdom. 



The 'Kolbenzellen' organ present in both BrancJiipus and Limulus 

 represents a degenerate pair of compound eyes.^ Their situation 

 and structure allow them to be compared to the hypostomic eyes 

 of Trilobites. In the living groups, however, they are in a more 

 retrogressive condition than in Trilobites. They never show any 

 trace of lenses, and in late development fuse and sink beneath the 

 ectoderm. 



Patten, in a discussion of the homologies of the 'Kolbenzellen' 

 organ in Brancliipus and Limulus, thought that the homology of 

 these would furnish very strong evidence of the common ancestry 

 of Crustacea and Arachnida in the Trilobites. We see that the 

 hypostomic eyes of Trilobites, which were of course unknown to 

 Patten, offer no support for this suggestion. They are already in 

 a degenerate condition, and are inherited from some ancestor which 



^ Patten, " On the Morphology and Physiology of the Brain and Sense Organs 

 oiZimulns": Q.J.M.S., July, 1893. 



^ W. K. Spencer, " Zur Morphologie des Centralnervensystems der Phjdlopodeu 

 nebst Bemerkimgen iiber deren Frontalorgane " : Z. im- Wiss. Zool., Ixxi, 3. 



