PROBOSGIS OF SYLLIDEA 329 
described here. They both become specially moditied towards 
the anterior end of the organ m connexion with the valvular 
apparatus to be deseribed later. 
V. ‘ue Proventrricutus : Muscunar KLEMENTS. 
The greater part of the substance of the thick wall of the 
proventriculus (figs. 1-5) is made up of the radial musele- 
columns and the annular bands. he former are hollow fibres, 
squarish or polygonal in cross-section, arranged in annular 
rows, and extending radially from the outer fibrous membrane 
to the inner. 
The hollow of each column is occupied by a protoplasmic 
core. In the columns which are perforated by the annular 
bundles the protoplasm is divided into anterior and posterior 
halves, and this division may extend to the mner end, but not 
to the short portion of the core outside the annular bands, 
the two halves being here continuous. In the Exogoneac 
and in certain members of the other groups each core contains 
only a single nucleus. But in the rest the structure is more 
complicated and the number of nuclei increased. The maximum 
of complexity is reached in the case of Syllis coruscans. 
In this species (fig. 4), in which the arrangement of the muscles 
is of type 2, the core is permeated by a system of exceedingly 
fine fibrils—forming an irregular meshwork with a prevailing 
longitudinal arrangement : this is more condensed towards the 
outer end. Communications occur between adjoining cores of 
the same row along the lines of the annular bands, and there 
are also communications, irregularly arranged, between the 
columns of neighbouring rows by means of processes which 
perforate the cortex. Fibrils from the meshwork of each core 
radiate outwards and penetrate through fissures into the sub- 
stance of the cortex. Such communications are most numerous 
opposite the Z membranes (Krause’s membranes) of the cortex, 
if they are not entirely restricted to such an arrangement. 
Nuclei are present in large numbers in each core. These 
are of two main varieties—larger, clearer nuclei of about 
(:0075 mm. in diameter, and smaller, denser, of a diameter of 
