934 SUMMARY OF CUnRENT RESEARCHES RELATING TO 



they nro tlio homologncs of the pleural ganglia of Gastrojiods. The 

 ccrcbroplcural coiuniissure in Dcntaliiun is very short ; the cercbropcdal 

 and the plouroiicilal commissure run together for almost their whole 

 course, and appear indeed to be closely fused with one another. Tho 

 author cannot confirm Fol's statement that the ganglionic cells are all 

 unipolar, and in all the nerve-fibres he finds scattered nuclei. 



Both Lacazc-Duthicrs and Fol have failed to notice that there are 

 two kinds of tentacles, which may be distinguished as " true " and 

 " rudimentary " ; the former are placed on the outer and the latter on the 

 inner side of the lamolliform fold, which lies beside the cerebral ganglia ; 

 the former arc, moreover, very long, hollow, and contractile, and have a 

 subepithelial muscular layer, which extends through the whole length 

 of the tentacle ; internally there is a nerve which swells out at the 

 widened end of the tentacle into a ganglion. Nerves, muscles, and cilia 

 arc absent from the short solid rudimentary tentacles. There arc some 

 intermediate stages between these tsvo kinds of tentacles. Sj^ecial 

 sensory organs are found in the end-bulbs of the true tentacles, in tho 

 form of about a score of long richly granular cells of a nervous cliaracter. 

 They are continued into a filament which swells out into an elongated 

 club just in front of the elongated pit which is found on tho ventral 

 surface of the terminal bulb. The thick end of this passes through tho 

 cuticle and carries a number of small sensory rods. This is a form of 

 tactile organ which does not seem to have been before observed in the 

 Mollusca. 



The otocysts have a low epithelium which carries a number of 

 isolated tufts of cilia ; the auditory nerve spreads out on the outer side 

 of the epithelium, and, on the other side, soon fuses with the commissure 

 which extends from the pedal ganglion to the nervous centres above the 

 oesophagus. 



The epithelium of the two lateral pouches in the oral cone differs 

 from that of the true buccal tube only in the want of cilia ; these diver- 

 ticula must consequently be regarded as labial pouches and not as 

 salivary glands. No suggestion can be offered as to the function of the 

 glandular swelling on the rectum, which contains on its better developed 

 side a multiramified ca^cal process of the rectum, which is lined by a 

 ciliated epithelium. 



The renal tube of one side has a much wider lumen than that of the 

 other ; neither has an internal orifice. The walls consist of a simple non- 

 ciliated epithelium, and their secretion is granular. Fol is correct in 

 saying that there is no special efferent duct for the gonads, the products 

 of which make their way to the exterior through the kidneys. The head 

 of the spermatozoon is divided into a long median piece and two short 

 terminal pieces. The tail is long and extends through the whole length 

 of the head in the median line. 



Tliere are no vessels, sinuses, or lacuna) from a histological point of 

 view, blood-spaces with true walls being completely wanting. The 

 structure of the " water-opening " on either side of the anus does not 

 seem to have been correctly apprehended by previous workers. The 

 epithelium of the body-wall is, at these points, arranged in several 

 layers, and forms on either side a slight elevation, the cells of which 

 have a remarkably clear protoplasm ; a few muscles are connected with 

 the oriBco. In the living animal these orifices are ordinarily kept 

 closed, and are only suddenly opened and as suddenly closed. There is 



