460 PROF. OWEN ON THE ANATOMY 
the thorax, seven for the abdomen, and, admitting the same number (as indicated by 
sense-organs and appendages), seven for the head,—total twenty-one. Now this cha- 
raeter could not be predicated of the Entomostraca ; some had more, some fewer segments. 
Branchipus stagnalis, for example, had eleven thoracic and nine abdominal segments, 
besides the head, protected by its cephalic shield. In Jsawra, in which this shield is also 
present, and of great size, the number of thoracic and abdominal segments exceeded | 
twenty-four. : 
Amongst the Trilobites the part of the body next the shield-shaped cephalie one shows 
eight segments in Asaphus platycephalus, eleven segments in Phacops, and from thirteen 
to fifteen in Calymene, besides an abdomen of eight segments. Then there were departures 
in Entomostraca from the Malacostracous numerical or segmental character by defect as 
well as by excess,—forms, like Limulus, e. g., with less than twenty-one segments. 
Moreover “the trilobed character of the segments in Serolis and Bopyrus is present 
also in Limulus, the segments of the hinder division of its body presenting three eleva- 
tions or lobes. The eyes, it is true, are large, sessile, and compound in Cymothoa; but 
so are the larger pair in Limulus, and more like those of the Trilobite than the eyes of 
any Isopods are; the larval Limuli, moreover, roll themselves into a ball" *. 
The value of the numerical character of the segments of the body in the question of 
the affinity of the Trilobites was pointed out by me in a lecture on Crustacea published 
the week after its delivery, April 27th, 1843 +. ij 
Burmeister, whose excellent work on Trilobites appeared at Berlin in a later part of 
1843 1, insisted, with equally original views, on the importance of this character; but, 
for his remark that ** Limulus was still more widely removed from the Trilobites than 
the Isopods are " $, I could not see adequate grounds. 
All this, however, is now of mere historical interest; and I fully. concur with my 
experienced colleague, Henry Woodward, Esq., F.G.S., whose labours have shed so 
valuable a light on the affinities and homologies of the Crustacea other than those in 
which “the normal number of segments is twenty-one,” that “the conclusions of Prof. 
Agassiz and James Hall as to the close affinity existing between the Eurypterida and the 
Xiphosura are correct"|. Whether the extension of Dana's group, Merostomata, as 
* Owen, *Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals, ed. 1855, 
p. 331. 
T In the following terms :—“ The distinction between the Entomostraca and Malacostraca in the number of the 
segments of the body is of the first importance in determining the affinities of the ancient extinet Crustacea called 
* Trilobites?” ‘Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals,’ 8vo, 1843, 
p. 165. 
i A translation of this work, with notes, by T. Bell & Ed. Forbes, was published by the Ray Society in 1846. 
$ In this view, however, Burmeister received the support of Emmerich in Leonhard und Bronn's Neues Jahrbuch, 
1845, part i., translated in * Taylors's Scientific Memoirs,’ vol. 4, part xiv. p. 253, August 1845. Emmerich defines 
the * Trilobites as a peculiar order, connecting Malacostraca with Entomostraca, but nearer the latter. They are 
related to the former by their calcareous crust-like shell, and by their not possessing simple eyes in conjunction with 
compound eyes. The Woodlice (Isopoda) have, of all Malacostraca, the greatest resemblance to Trilobites.” 
| ‘ Monograph of the British Fossil Crustacea belonging to the order Merostomata, Part I’, Palæontographical 
Society's vol. for 1866, p. 9. 
