34 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 21, 



In the development of the embryo of any of the Crustacea, 

 whether before or after exclusion from the egg, the segments which 

 form the body are developed in succession from before backwards ; so 

 that when their evolution is checked, the posterior rather than the 

 anterior rings are those that are wanting ; and, in fact, it is gene- 

 rally easy to see in those specimens of fuU-gTOwn crustaceous 

 animals whose bodies present fewer than 21 segments, that the 

 anomaly is due to the absence of a certain number of the most pos- 

 terior rings of the body, [See Milne-Edwards on the development 

 of Crustacea.] 



So long ago as 1838, Dr. Milne-Edwards * gave an account of an 

 examination of the young of Limulus at the time of its extrusion 

 from the egg. He found that, as regards the anterior portion of their 

 bodies, they presented little diiference from the adult; but the 

 posterior portion had only three pairs of appendages ; and the long 

 styliform telson, so remarkable in the adult, did not exist at all. 

 [See PL II. figs. 3 & 4.] 



It is, then, I think, reasonable to assume that the absence of the 

 normal number of segments in Limulus is due to the abortion or 

 non- development of the hindmost abdominal somites rather than 

 of those belonging to the other divisions of the body. 



As to the union of two dissimilar parts, so as to leave no indica- 

 tion of their blending, there are numerous eases in the Cyclopoidea, 

 as well as in other orders. In Cyclops the ovarian segment is 

 double, being formed by the coalescence of the last thoracic and the 

 first abdominal somites. In Corycceus crassiusculus eight segments 

 unite to form the head, as is the case in Limulus~i. e. seven cephalic 

 and one thoracic somites. 



Taking the Eurypterida first, we find five genera and one sub- 

 genus, having a head bearing seven paired appendages, and (adopt- 

 ing Professor Huxley's view, that the antennules or inner antennae 

 are absent) representing eight segments {i. e. seven cephalic and one 

 thoracic). Add to this the twelve free body-segments and the 

 telson, and we have twenty-one, or the normal number of somites. 

 With the exception of the two most anterior segments (which pro- 

 bably had branchial appendages attached to their under surfaces, 

 concealed beneath the operculum or thoracic plate), all the free 

 somites are destitute of limbs of any kind ; and in this respect they 

 strongly resemble the larvae of the Decapoda f. 



In Remiasjpis we have the body composed of a head, six thoracic 

 and three abdominal somites, and a telson. 



In Exapinurus and Pseudoniscus of a head, five thoracic and four 

 abdominal somites, and a telson. 



In Belinurus, a head, five thoracic and three abdominal somites, 

 and a telson. 



In Prestwichia, a head, five thoracic and three abdominal somites, 

 and a telson. 



* " Sur le d^veloppement des Limules,'' Societe Philomathique, Nov. 10, 1838. 

 t See Mr. C. Spence Bate " on the Development of Decapod Crustacea," Phil. 

 Trans. 1858, p. 689, pi. xl. figs, a & b. 



