W. K. Spencer — Hypostomic Eyes of Trilohites. 491 



may very possibly have been an ancestor of the Arthropod group 

 in general. Patten wrote before the publication of the work of 

 Beecher and our consequent true understanding of the Crustacean 

 affinities of Trilobites.^ Further, the discovery of the uniform 

 occurrence of anterior chelate appendages throughout the Merostomata 

 removes the points of resemblances which were formerly~urged 

 between the fossil allies of Limulus and the Crustacea, for it can na 

 longer be argued that the first appendages of these were antenni- 

 form. PalEeontological evidence has thus widened the gap in two 

 directions, on the one hand by assigning the Trilohites definitely 

 to the Crustaceans, on the other by showing the unity of the 

 Merostomata. The embryological evidence of the Trilobite affinities 

 of Limulus is also very weak. The young Limulus in the so-called 

 ' Trilobite ' stage possesses no true biramose appendages, at any 

 rate on the cephalon, although a sense organ on the sixth pair has 

 been described as an exopodite. The appendages of the cephalon 

 are seven pairs, not the five pairs so characteristic of Crustaceans. 

 The Arachnida and Crustacea have always been distinct groups, 

 and must have been differentiated in pre-Trilobitan times. 



It has been suggested that Trilohites had much the same manner 

 of life as Limulus. The fact that so few Trilohites have been found 

 which show any trace of appendages militates against this view. 

 We know Trilohites such as Isotelus megistos, or even Paradoxides 

 Davisii, which are of very considerable size. If these had had legs 

 stout enough to walk with, some trace must have been observed. 

 Beecher has shown from positive evidence that the legs of Triarthrus 

 are of extreme tenuity. We must assume, therefore, that the 

 appendages of Trilohites were incapable of affording support for 

 walking, and they must have been swimming appendages. Burnieister^ 

 long ago demanded for Trilohites a mode of life similar to that of 

 the Phyllopoda. He observed that the remains of Trilohites were 

 almost always found on their back. Beecher ^ observed the same, and 

 remarked with regard to Triarthrus Becld that the material shows 

 both legs and antennae extended on both sides of the body in a very 

 life-like position. This was a feature incompatible with the view of 

 Walcott that Trilohites lived with the ventral side down, and the 

 accumulation of gases in the viscera during decomposition overturned 

 the animals ; for this would lead to great displacement of the 

 appendages. The ventral food groove, identical in appearance 

 with that of Phyllopods, leads to the same conclusion. The 

 Phyllopods appear to feed by turning over whilst swimming and 

 seizing with their more posterior appendages a little mud which 

 swarms with Infusoria, etc. This mud is then pushed along the 

 ventral groove into the mouth. Casts of the intestines of Trilohites 

 are still found filled with the mud acquired in this manner. The 

 hypostome is obviously of great service in guiding the food in 



1 See H. "Wood-ward, Quart. Jouru. Geol. Soc, H (1895), 



2 Burmeister : ' ' Die Organisation der Trilobiten aus iliren lebeuden Verwandteu 

 entwickelt " ; Berlin, 1843. 



2 Beecher, " On the Structure and Development of Trilohites " : Amer. Geol., 

 xiii (1894). 



