746 SUMMARY OF CURRENT RESEARCHES RELATING TO 



can go on ; the operculum may perhaps have been formed by the 

 concrescence of a series of gill-branches arranged in a circle round 

 the tip of the gill-filament. 



In Sahella the gill-filaments differ from those of Serpula by the 

 presence of a cartilaginous rod and a portion of the longitudinal 

 muscle-layer of the body which is prolonged into them in close con- 

 nection with this skeletal rod. 



The paper concludes with a discussion concerning the homologies 

 of the gills in Serpulacese with the gills of Vertebrata, which is 

 believed to exist by some ; the author, however, does not con- 

 sider that there is any homology or even analogy between the two 

 structures. 



Structure and Development of Fresh-water Dendrocoela.* — The 



studies of J. Jijima are based chiefly on Dendroccelum ladeum, 

 Planaria polychroa, and Polycelis tenuis (n. sp.). Commencing with 

 a description of the cilia, he states that, in adult forms, these are not 

 developed over the whole surface of the body, but are absent from 

 certain regions ; they are particularly well developed at two points at 

 the anterior margin of the head, where they form a tuft of long and 

 constantly moving hairs ; their function would appear to be sensory. 

 Some shorter immobile cilia are found on the median portion of the 

 cephalic margin, and among these there are some which are twice as 

 long, and either stand separately or arise from a common base ; they 

 may be regarded as comparable to setae. The absence of cilia from 

 the sides of the body may be ascribed to the influence of parasitic 

 Protozoa. It would seem that the cilia on the back of the Geoplana 

 and other terrestrial Triclades, are more delicate than those on the 

 ventral surface, and are, therefore, more easily destroyed in the 

 process of preservation. 



The author was, like Kennel, unable to detect the unicellular 

 epidermal glands seen by Moseley, and is led to doubt their presence ; 

 a certain relation was observed between the rhabdites and the 

 characters of the cells of the epidermis ; the smaller size or number 

 of the former being associated with a greater wealth of finely glanular 

 protoplasm in the latter. Jijima finds that there is a very re- 

 markable relation between the cells and the basal membrane on 

 which they are placed ; for the former give off a number of fine pro- 

 cesses ; these are best studied in Planaria polychroa, where they appear 

 to be formed by fibrils, which are nothing else than direct proto- 

 plasmic processes of the cells of the epidermis ; there is little doubt 

 that there is an organic connection between the epithelium and the 

 interior of the body. 



The rhabdites, which are described in some detail, do not seem to 

 be imbedded in the epithelium, but in the peripheral cells of the 

 mesenchym. Each cell gives rise to several rhabdites, which are 

 at first small and round, but which soon elongate. When they have 

 reached their definite size they break through the cell-wall, which 

 appears at last to be absorbed, and wander through the connective 



* Zeitschr. f. Wiss. ZooL, xl. (1884) pp. 359-464 (4 pis.). 



