119 



OTted gills. The central axis is traversed lengthways hy a nerve trunk 

 and branches from the^ trunk pass out into each one of the lateral 

 plates. This may be readily seen in sections. 



In none of the figures or descriptions of Crepidula to which I have 

 had access is the osphradium noticed. Examination of the creature 

 proves however that the organ is present though in by no means the 

 usual form. My specimens were all of the species Cr. fornicata \j?cm. 

 The gill in this species-* has been described in a former paper as a 

 series of long blade shaped filaments which stretch across a low broad 

 mantle cavity. It occupies so much of the mantle cavity that but little 

 space remains upon the roof of the cavity. It is in the space remaining 

 which lies upon the left of the gill-ridge from which the filaments arise 

 that the osphradium is situated. There stand here 18 or 20 papillae in 

 a longitudinal row upon a low ridge parallel with the gill but much 

 shorten. Each papilla presents a globular expanded head supported 

 upon a short narrowed peduncle. It is light colored except upon the side 

 turned toward the gill where it is occupied by a large very dark spot. 

 The papillae stand somewhat further apart than the gill filaments. 



Examination by means of sections showed that the central ridge or 

 longitudinal axis of the organ is traversed by a nerve trunk and that 

 this sends a branch into each papilla. The papilla itself is covered with 

 epithelium which is columnar upon the sides and stands upon a well 

 defined basement membrane. Upon the side looking toward the gill 

 the cells are deeply pigmented. Upon the summit of the papilla the epi- 

 thelium cells are taller and broader and with a very indefinite outline 

 Furthermore there is no distinct basement membrane upon the summit 

 as on the sides of the papilla the lower ends of the cells being appa- 

 rently very irregular. The free ends of the cells further are apparently 

 ciliated; there is very little doubt of them being ciliated; the condition 

 of my specimen made satisfactory determination of this point impos- 

 sible. There seems to be but little room for doubt that these terminal 

 cells are those which are especially sensory. 



There is besides this organ an area of peculiarly modified epi- 

 thelium which runs along the ridge from which the gill filaments arise 

 and of Avhich no mention has been made hitherto. It forms the covering 

 of the side of the ridge which is turned toward the osphradium and 

 runs its whole length. It consists of very tall cells which are entirely 

 unlike any other cells upon the mantle and are so set as to form what 

 appears to be a specialized organ. 



Purdue University, T-afayette Indiana, 21. December 1886. 



3 Studies Biol. Lab. Johns Hopkins Univ. Vol. III. p. 42. 



