BHABDITE-CELLS IN OEPHALODTSCTTS. 265 



Finally it is necessary to mention another stage wbieli was 

 only of occasional occurrence in tlie material at my disposal, but 

 whicb seems to be common in tbe sections of Cephalodiscus in 

 tbe laboratory of tbe Eoyal College of Science. Here tbe 

 rhabdite-" cells " are sbarply divided into two parts, so as to 

 present a strong superficial resemblance to certain o£ tbe pseudo- 

 rbabdites described by Biirger in ISTeraerteans. Tbe upper or 

 apical portion contains a body whicb undoubtedly corresponds 

 to tbe secretion-sphere and its products described above. The 

 lower or basal portion is of a uniform, structureless and highly 

 refractile appearance, as if this portion of the " cell " were filled 

 with a homogeneous waxy secretion. I am unable to connect 

 this stage with the others with certainty, but it seems to be an 

 early stage in the formatiou o£ the rhabdites, and may possibly 

 be the earliest yet seen in the material. Its occurrence was of 

 too occasional a character in my material to enable me to study 

 it as carefully as the otlier stages. 



The question that now arises is from what source are the rhab- 

 dite-" cells " of Cephalodiscus derived. That they are portions of 

 cells, and not complete cells in themselves, must be admitted, in 

 view of the fact that they do not possess any structure that may 

 be justifiably interpreted as a nucleus. On tbe other hand, 

 their position and mode of occurrence is strongly suggestive of 

 the view that they are disassociated portions of the basal cells 

 (fig. 1, -^) of the battery. These basal cells therefore, on this 

 view, will be constantly giving rise to the so-called rbabdite- 

 " cells," and tbe latter will as constantly be discharged on to tbe 

 exterior. The term rhabdite-cell will hence belong properly to 

 the bodies described in this paper + also tbe basal cells of the 

 battery, since the two together are tbe morphological equivalent 

 of the Turbellarian rhabdite-cell and its contents. 



In order to justify tbe term I have applied to the structures here 

 described, tbe following comparison is appended between these 

 bodies and tbe cells in Turbellaria to which tbe term rhabdite- 

 cell was first applied by Graff in 1882. Lang * in the Polyclads 

 distinguishes two kinds : (1) Rbabdite-cells ; and (2) " Scbleim- 

 stabchenzellen " or Pseudo-rhabdites. Of the former be says 

 (pp. 51-52) : — " Der Kern der Stiibchenzellen liegt stets am 

 basalen Ende derselben ; das freie distale Ende der Zellen ist mit 

 Elimmerbaaren besetzt, wovon man sich durch Isoliren der 

 * Fauna u. Flora d. Grolfes v. Neapel, Monog. xi. pp. 51-55. 



LINN. JOUEN. — ZOOLOGY, VOL. XXVII. 20 



