Affinities of Trilobites. 1 7 



treille, but also his Lcemodipoda and Isopoda,) or whether they 

 belong to the Entomostraca or those existing Edriophthalma 

 which do undergo a change of form in their larva state. I 

 conceive that the Trilobites will be found to differ in so many- 

 respects from both the Amphipoda and Entomostraca, that ac- 

 cording to the present state of our knowledge, we must allow 

 them to form a distinct order, intermediate between the tribe 

 Isopoda on the one side, and the tribe Aspidophora on the other. 



Those circumstances which generally are reckoned most 

 anomalous in the Trilobites are not in reality so very extraor- 

 dinary, since they may be detected in many Crustacea now 

 existing. Thus the trilobed form of the body occurs in Serolis 

 and Bopyrus. The membranaceous or rather coriaceous mar- 

 gin of the body, assumed by Latreille and others to exist in 

 Trilobites, is to be found in the female Cymothoce. In these 

 last animals also, as well as in the female Bopyrus, we observe 

 the eyes to disappear as in many Trilobites. The compound 

 eyes of Calymene are situated on the back of the head, but wide 

 apart, and are composed of large facets. The same structure 

 may be seen in the male of Cymothoa trigonocephala, and 

 many other Cymothoadce. The absence of antennae and the 

 rudimentary state of the feet, both occur in Bopyrus, the well- 

 known parasite of prawns. In Spheroma we have not only 

 the onisciform body of Calymene, but also its property of roll- 

 ing itself up into a ball. In Spheroma also we find the large 

 convex semicircular anal segment of Bumastus. I think, there- 

 fore, that we can have no hesitation now in allowing the im- 

 mediate affinity of the Trilobites to Isopod Amphipoda, and 

 more particularly to the Cymothoadce and that parasitical 

 group which is called Epicarides by Latreille. Indeed, if the 

 Trilobites are once demonstrated to have possessed articulated 

 feet, it will be difficult to remove a male Bopyrus from the 

 group. Here the two eyes are placed on the back of the head 

 wide apart. Here also there are no antennae, no posterior 

 lateral abdominal appendages, and besides no very distinct 

 articulation to the sternum. If the Bumastus of Murchison 

 had a body of thirteen equal segments with short crustaceous 

 feet it would be a male Bopyrus, so close is the affinity ! The 

 differences between a male and female Bopyrus, such for in- 



Ann. Nat. Hist. Vol.4. No. 21. Sept. 1839. c 



