628 PROFESSOR E. RAY LANKESTER, 
briefly summarise their agreements with Arachnida and 
disagreements with Crustacea as follows: 
1. Limulus and the Eurypterines (the one supplementing 
the other) agree precisely with the Scorpion in the existence 
of eighteen segments expressed in the structure of their 
bodies, and in the distribution of these segments into three 
groups of six each, viz.: a leg-bearing cephalothoracic region, 
an anterior abdominal region, in which each segment carries 
lamellate appendages, anda posterior abdominal region devoid 
of appendages, ending with the anus and a postanal spine. 
No Crustacean presents this number and grouping of its con- 
stituent somites. 
2. Limulus and the Eurypterines agree with the Scorpion 
precisely in the position of the genital aperture beneath an 
opercular plate formed by the coalescence of the seventh pair 
(in Eurypterines the actual sixth pair of appendages). No 
Crustacean has the generative orifice so far forward, and in 
none is there a genital operculum of the kind having such 
relations of position to the genital apertures. 
3. They agree with the Scorpion in the character and 
position of the mouth and upper lip. 
4. They agree with the Scorpion in possessing a meta- 
thoracic sternite, in the possession of a fibro-cartilaginous 
entosternite, and in the precise form and relations of that 
organ. No Crustacean possesses an entosternite or any 
structure resembling it. 
5. They agree with the Scorpion in the disposition of 
central (single) and lateral (grouped) eyes on the cephalo- 
thorax. No Crustacean has an identical arrangement of 
single and grouped eyes. 
6. Limulus agrees with the Scorpion in the form of the 
alimentary canal and its lateral outgrowths (liver), which are 
more than one pair. In Crustacea it is very exceptional to 
find more than one pair of such diverticula, though a single 
pair may carry numerous secondary branches. 
7. It agrees with the Scorpion in possessing a supra- or 
circum-medullary (spinal) artery, which arises from the dorsal 
aorta by two arches embracing the esophagus. No Crus- 
tacean has such a supra-spinal artery so originating. 
8. It agrees with the Scorpion in the form of the genera- 
tive glands. No Crustacean has its generative glands in the 
form of an anastomosing network. 
9. It agrees with Scorpio in possessing vibratile sperma- 
tozoa. No Crustacea except Cirrhipedia are known to have 
vibratile spermatozoa. 
10. It agrees with Scorpio and Spiders in having a brain 
