232 THE ANATOMY OF INVERTEBRATED ANIMALS. 



the post-oesophageal ganglia, internal to the nerves of the 

 seventh pair of appendages. The longitudinal commissures 

 are very long, and are inclosed in a continuation of the same 

 sheath ; they pass back into the second division of the body, 

 and there present four ganglionic enlargements, whence the 

 nerves of the post-opercular appendages proceed. The last 

 of these ganglia is much larger than the others, and appears 

 to consist of several confluent masses. The nerves diverge 

 from it in such a manner as to resemble a cauda equina. 



The reproductive organs of both sexes consist of a mass 

 of glandular cseca, which ramify through the body amid the 

 hepatic tubules, and eventually open on papillae situated on 

 the posterior face of the operculum. The males are much 

 smaller than the females, and present, in many species, an 

 external sexual distinction in the peculiarity of their second 

 and third appendages already referred to. 



The young of Limulus acquires all its characteristic 

 features while still within the egg. The interesting obser- 

 vations of A. Dohrn 1 have shown that, in an early stage, the 

 embryo is provided with the nine anterior pairs of append- 

 ages, and is marked out into fourteen somites by transverse 

 grooves upon its sternal face. The body has the form of a 

 thick rounded disk, divided into an anterior shield composed 

 of six somites, and a posterior, likewise shield-shaped region, 

 formed by the union of eight somites. The telson has not 

 made its appearance. In this condition, its resemblance, 

 apart from the limbs, to such a Trilobite as Trinucleus is, as 

 Dohrn points out, most remarkable. 



The Xiphosura were represented in the Carboniferous 

 epoch {Bellinurus). 



The Eurypterida (Fig. 59) are extinct Crustacea of Pa- 

 laeozoic (Silurian) age, which sometimes attain a very large 

 size and in many respects resemble Limulus, while, in others, 

 they present approximations to other Crustacea, especially 

 the Copepoda. An anterior, eye-bearing, shield-shaped di- 

 vision of the body is succeeded by a number (12 or more) 

 of free somites, and the body is ended by a broad, or narrow 

 and spine-like, telson. Five pairs, at most, of limbs, pro- 



1 " Untersuchungen uber Ban und Entwickelung der Arthropoden." (Jena- 

 iscTie Zeitsclirift, Bd. vi.) See also the observations of Lockwood and Packard, 

 American Naturalist, vol. iv., 1871, vol. vii., 1873, and " Memoirs of the Boston 

 Society of Natural History," 1872 ; with the discussion of the systematic place 

 of Limulus by E. Van Be'neden, Journal de Zoologie, 1872. 



