Mr. A. Tulk on the Anatomy uf Plialangium Opilio. 325 



their external border to the remaining extremities. Three branches 

 arise from the posterior margin of the transverse portion, to sup- 

 ply the contents of the abdomen. A medio-abdominal nerve (m), 

 which passes beneath the transverse tracbeal vessel, and above the 

 ovarium and ovisac in the female and seminal vessels in the male, 

 and divides into two branches [ag'), furnished with pyriform 

 ganglia of considerable size. Beyond these ganglia the branches 

 are continued for a short distance, when they unite by a transverse 

 filament (* *) and subdivide into delicate nervous threads, few in 

 number, which again communicating, form an open net-work of 

 nervous fibre, distributed to the ovarium, oviduct and tissue of the 

 corium. The two external or latero-abdominal nerves divide each 

 near to their origin into two others, the outermost of which [gg], 

 very short, presents occasionally two gangliform swellings upon 

 its course, and gives branches to the generative organs near to 

 their external opening. The inner pair {ag) pass backwards almost 

 parallel with the central cord, and provided also with ganglia di- 

 vide, according to Treviranus, into three filaments which spread 

 out upon the under surface of the alimentary canal and adjacent 

 viscera. The thoracic ganglion is in relation in the middle, above 

 and posteriorly, with the transverse tracheal vessel, laterally with 

 the main tracheal trunks, and beneath with the termination of the 

 ovipositor (PI. V. fig. 33. t). 



It was important to ascertain if, in the structure of the gan- 

 glionic centres of the nervous system in Plialangium, there was 

 anyindication of a separation into two distinct tracts, such as have 

 been described in insects as representing the motor and sensitive 

 columns from which the spinal nerves arise in the A^ertebrata. 

 With this view I have examined under the microscope the tho- 

 racic as well as the abdominal ganglia, and find that in both they 

 are composed of an aggregation of globules, very distinct in the 

 first, where they resemble somewhat vesicles of fat, which have 

 coalesced together so as to form an irregular kind of net-work. 

 The nervous fibres which pass either to or from these ganglionic 

 enlargements appear to cease abruptly when they arrive at the 

 latter, and become, as it were, amalgamated with their structm-e. 

 I mention briefly these facts here because they offer additional 

 evidence, to that which has already been so admirably adduced by 

 Prof. Owen in his Lectures upon the Crustaceaf during the pre- 

 sent year, of the falsity, or, at least, non-universality, of the above 

 analogy, as applied to the Articulated animals. The most striking 

 peculiarity connected with the nervous system of the Phalangia, 

 is the presence of several large transversely striated muscular 



t Lectures on the Comparative Anatomy and Physiology of the Inverte- 

 brate Animals, 1843. 



