486 MR. F. E. BEDDARD ON THE 
ments of the muscular fibres showing various intermediate stages between 
the bipinnate muscular bands of Lwmbricus and the fibres of Urocheta, which 
are present in a continuous mass without any dividing septa of connective 
tissue. In Lwmbricus the longitudinal muscular layer may be regarded as 
being composed of a series of compartments formed of trabecule of fibrous 
tissue, in the interior of which lie the actual muscular fibres. On Plate XX VI. 
fig. 10 is a diagram of this; the fibres are developed close to the septa 
themselves, and thus give rise to the bipinnate arrangement so well dis- 
played in the drawings of CLApAREDE. In Pleurochwta a distribution of 
the muscular fibres exactly like this does not exist, but in the anterior 
segments the longitudinal coat is divided in a precisely similar manner into 
compartments, only that there are more muscular fibres in each compartment, 
and they are not all developed close to the septa. In a young specimen not 
more that 14 inches in length, which I had the opportunity of examining, 
and which will be described in the last part of this memoir, a section through 
the anterior end of the body (Plate XX VI. fig. 15) showed the muscular fibres 
arranged in vertical lines, which were more thickly congregated in the neigh- 
bourhood of the septa, and possessing therefore an “orientation deter- 
minée,” differing only in degree from that of ZLumbricus. In the adult 
Pleurocheta sections through the body wall in the anterior region show a 
somewhat similar arrangement; the longitudinal coat, which is here rather 
more than double the thickness of the circular coat, is divided into com- 
partments, bounded by very thick bands of fibrous tissue, the interior of each 
compartment being again subdivided by other trabecule ; between each 
pair of compartments (Plate XX VI. fig. 13) there is frequently a space for the 
insertion of the lower end of a seta, in which only delicate fibres of connective 
tissue are visible : in this case, however, the muscle fibres have no fixed and 
definite arrangement, and they are not specially developed at the margin of the 
compartment. In the hinder part of the body the longitudinal muscular layer is 
not divided up, but here and there (Plate X XVI. fig. 4, a) a stronger trabecula 
serves to point out the boundary of regions which correspond to the anterior 
compartments ; here too, as already stated, the development of fibrous tissue is 
much slighter than in the anterior part of the body ; a comparison of figs. 4 
and 13 will make this clear. Finally, in the figures of Urochwta, given by 
Perrigr, the longitudinal muscular layer (as well as the circular) is entirely 
without this fibrous network. 
PERRIER, in his account of the anatomy of Pontodrilus, in which animal 
the longitudinal muscles are arranged in a manner similar to that which has 
already been described as occurring in the posterior region of the body wall of 
Pleurocheta, comparing the account given by CLAPAREDE for Lumbricus with 
his own description of Pontodrilus, says (page 186), “ La difference essentielle 
