MINUTE ANATOMY OF LIMNOCODIUM. 543 



porting lamella of the velum. Of the two divisions of the 

 nerve, the outer division on the side next the nettle-ring is 

 the most strongly developed of the two. Its fibres are per- 

 fectly well demarcated from the surrounding cells, and are 

 always easy of observation. They seem, moreover, to be en- 

 closed in a sheath or neurilemma. On the other hand, the 

 fibres which constitute the inner bundle of the nerve-ring 

 C^unterer Nervenring'' of Hertwig) are more difficult of certain 

 demonstration, since they are not distinctly separate from the 

 bases of the ectoderm-cells of the region. 



Sense-organs. — Concerning the velar sense-organs, mar- 

 ginal bodies, or refringent bulbs, I have nothing to add to 

 Professor Ray Lankester's description. I can only confirm 

 his account of these most remarkable organs in every particu- 

 lar. They consist of a small almost spherical multicellular 

 refringent body, which in the fully developed organ is attached 

 by a thin stalk near the nerve-ring, and is suspended in an 

 elongated sac which is embedded in the thickness of the meso- 

 gloea of the velum. The general relations of the organ are 

 shown in PI. 40, fig. 3, while in fig. 6 are several views of 

 sections of refringent bulbs carefully drawn with the camera 

 lucida. The refringent bulbs consist of cells of two kinds. 

 The more peripheral ones — the '' cortical cells ^' of Lankester 

 — are thin, and often so much stretche.d over the more central or 

 " medullary cells '' that they are difficult of observation . These 

 cortical cells are of ectodermal origin, and are continuous 

 with the lining of the sac in which the entire organ is enclosed. 

 The medullary cells impart the highly refringent appearance 

 to the bulbs, and are of endodermal origin, being budded off 

 from the lining of the circular canal, and subsequently become 

 completely enclosed by ectoderm. In the young condition 

 these medullary cells have a granular appearance (PI. 40, 

 fig. 6, a, med. c.) like the cells lining the circular canal, and 

 only become clear and refringent as the bulb approaches 

 maturity. Even in fully developed bulbs a few of the medul- 

 lary cells near the point of attachment of the bulb still retain 

 a certain granular character (fig. 6, d, e) . 



