W. K. Spencer — Hypostomic Eyes of Trilohites. 489 



III. — The Hypostomic Eyes of Trilobites. 



By W, K. Spencer, B.A., F.G.S. (Burdett-Coutts Scholar in the University 



of Oxford). 



PEOFESSOR LINDSTROMi in a recent paper has described 

 sense organs discovered by him on the hypostome oF very 

 many genera of Trilobites. His description is as follows : — 

 *' In the pluralit}'' of species there are two tiny patches or macule, 

 sometimes elevated above the surrounding surface like tubercles, and 

 so they have also been called by some authors. But I have preferred 

 to use the name ' macula ' for them, as the plurality does not form 

 tubercles. They are generally smooth and glossy, and situated next 

 to the anterior groove, either above it or in it, at a regular distance 

 from each other and the lateral margins. They may form a sunk 

 spot, or, as commonly, an ovoid or elliptic area surrounded by 

 a linear elevated border." "Common for a great number of maculae 

 in various groups, whether they show any organic structure or not, 

 is the excessive thinness of their shell in comparison with that of 

 the surrounding hypostoma. This is also in accoi'dance with the 

 tenuity of the cephalic eyes in relation to the test of the cheeks." 



Beyond the •' excessive thinness of their shell " the maculae of 

 the various genera show scarcely any constancy in their structure. 

 A section through the macula of Bronteiis shows distinct traces of 

 lenses on the anterior apex (Fig. 2). "Only in the Asaphidse, in 

 Illcenus and Lichas, the entire macula shows this structure. Perhaps, 

 to judge by certain indications in Bronteus, once in a larval or pre- 

 ceding stage of evolution the whole surface of the macula was also 

 in that genus covered with lenses, which have been reduced." 



In other genera, e.g. Biimastus and Nileus, there is no trace 

 whatever of any structure. " There is even in the same genus so 

 great a variability that species with structure in the macula occur 

 along with those devoid of any structure, or also, as in Lichas, with 

 a different structure." 



A great diversity in position may also occur. In Phacops and 

 Acaste the maculee are high up on the hypostoma, near its anterior 

 margin. 



The great variability in structure and position, together with the 

 obvious stages of reduction, teaches us that the ' hypostomic eyes ' 

 of Trilobites are in a degenerate condition. My object in writing 

 this paper is to show their close correspondence with certain sense 

 organs present in the Phyllopod genus Branchipus, and also in 

 Limulus. In Branchipus there occurs, ventral to the brain and just 

 anterior to the hypostome, a median organ which Glaus ^ has called 

 the ' Kolbenzellen ' organ. This consists of a number of nests of 

 cells. Each nest (Fig. 3) has secreted a rhabdom, just as occurs 

 generally in the compound eyes of Arthropods. Although it is 



1 G. Lindstrom, " Researches on the Visual Organs of the Trilobites": Kong. Sv. 

 Vet.-Akad. Hand., B. xxxiv, No. 8. 



* C. Glaus, " Uutersuchungeu iiber die Organisation uud Entwickelung von 

 Branchipus und Ariemia" : Arb. aus deni Zooi. Inst, in Wien, vi ('1885). 



