12 Prof. Mcintosh's Notes from the 



base to tlic tliioads in tlic delicate pinnne, ^nIucIi liavc a 

 thick coat of liypodeiin and a ciliated cuticle. In the young 

 Bispira the chordoid cells are especially large and distinct. 



The branchial skeleton thus springing from a firm base 

 spreads forward (or, as usually described, "upward'^) as a 

 vase- or funnel-shaped sheet, binding together the bases of 

 the filaments and, finally, dividing into the isolated rods for 

 the filaments and pinnules. At the origin of the filaments the 

 skeletogenous tissue forms abroad belt, continuous externally 

 as a narrow rim, and having within this a small group of 

 the chordoid reticulations, then a series of skeletogenous 

 areas (in section) sometimes with marginal muscles, indi- 

 cating the rudiments of the filaments. The chordoid 

 reticulations then become more numerous, the " perichon- 

 dria!" area diminishes, the soft parts inci-ease, and by-and-by 

 the separate filament is evolved. The chordoid rods to the 

 pinnules appear to pierce — if such an expression can be used 

 in connection with this continuous tissue — the "perichon- 

 dria!" investment of each filament, and come into contact 

 with the reticulations at the outer part of each. The whole 

 chordoid slieleton is^ however, a continuous structure, and 

 it is only the continuity of the areolae of tlie pinnules with 

 those of the filaments whicli makes tlie use of the term 

 " piercing the perichondrium " intelligible. A comparison of 

 the adult and young specimens of the annelid show that the 

 nuclei are remarkably distinct in the latter, whilst the 

 smaller number and proportional larger size of the cells are 

 features of moment. JNlany previous authors having used, 

 in connection with this skeletogenous tissue, terms which 

 would imply se])arate tissues, it has been necessary to insist 

 on the unity of the structure as a whole. 



Another feature of the chordoid skeleton is its connection 

 with the shedding of the whole branchial apparatus in the 

 Sab'ellids, for all the chordoid tissue appears to be thrown 

 oil" with the branchial fans and the tentacles, the funnel- 

 shaped anterior or distal portion consisting largely of this 

 tissue covered by the integuments. The vessels on the 

 proximal side would thus be more readily constricted, and 

 an active surface for the reproduction of the apparatus 

 uncovered. Whether this shedding of the branchial fans 

 occurs frequently in natuie is an open question, but the 

 annelids in confinement sometimes do so. 



The branchial fans double inward at their ventral base as 

 a thin lamina with miniature filaments, each with its chor- 

 doid axis, and along the inner border of each the nerve- 

 strands occur. 



I 



