220 SUMMARY OF CURRENT RESEARCHES RELATING TO 



visual cells to those that are pigmented, but Patten, on the other hand, 

 states that in Annelids, Arthropods, and Molluscs, tho visual cell never 

 contains pigment ; the axial nervous filament which the last-named writer 

 always finds in visual cells has not been detected in Branchiomma. The 

 eye of the Annelid may be regarded as being formed of two layers, 

 the more superficial of which furnishes tho dioptric apparatus, while the 

 lower gives rise to the sensory elements. 



Larval and Definite Excretory Systems in Lumbricidse.* — Prof. 

 F. Vcjdovsky has examined the larval stage of seven Hungarian species 

 of Lurabricidae, and finds in all these common characteristics : looked at 

 from above or below the larvae are more or less ovoid, ellipsoidal, or 

 spherical ; the unilaminate epiblast is ciliated on the ventral surface, 

 and the larvae are thereby enabled to execute more or less lively rotatory 

 movements in the albuminous fluid ; the anterior end is distinguished 

 by three (more rarely four or five) larval cells ; these have been hitherto 

 incorrectly called " Schluckzellen," but they must be regarded as con- 

 tractile epiblast-cells belonging to tho larval excretory system; they 

 arise very early, and some species may be recognized during segmenta- 

 tion by their intracellular network of canaliculi ; later on they are over- 

 grown by smaller epiblast-cells, and come to lie between the epi- and 

 hypoblast. Fine ciliated canaliculi are connected with these gland- 

 cells ; in Lumbricus rubelhs there is generally only one pair of these 

 excretory canaliculi ; the excretory fluid is gradually collected in the 

 intracellular ducts, which loop in various, but no doubt definite fashion, 

 and the clear fluid is, by a sudden contraction, expelled to the exterior 

 through a dorsal orifice. The larval canaliculi have begun to function 

 at the time when the two large mesoblasts begin to divide, and they are 

 consequently undoubted derivates of the epiblast. 



The remnant of the blastopore goes to form the stomodaeum ; below 

 it the anterior ends of the germinal stripes are united ; these now grow 

 on either side of the stomodaeum, and so give rise to the first segment. 

 The multiplying elements of this segment gradually press the glandular 

 cells of the larval excretory apparatus a little backwards into the median 

 dorsal line. The larval excretory canaliculi and the contractile gland- 

 cells do not disappear until the second and third segments are completely 

 developed. 



Independently of these larval excretory organs, a pair of straight 

 non-ciliated excretory canals become developed in the dorsal ccelom of 

 the first segment ; these are what the author has called the embryonic 

 or provisional excretory organs. They degenerate without leaving any 

 vestiges, while in the succeeding segments the excretory organs become 

 developed. These arise by the increase in size and division of a pair of 

 mesoblast-cells on the posterior side of the dissepiment of each segment, 

 which grow into a short solid cord; this very rapidly grows out into 

 a large group of cells, which make the exact study of the process of 

 nephridium-formation of Lumbricidae very difficult to follow. 



In young forms of Rhynchelmis, however, it is possible to follow out 

 the process step by step, for after the digestion of the yolk-elements the 

 worm is quite transparent. Each nephridium passes through a remark- 

 able cord-like stage which may be called the pronephridium ; after the 

 formation of the solid cord of cells a large cell arises anteriorly which 



* Zool. Anzeig., x. (1887) pp. G81-5. 



