496 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of the wall of the cavity, and their epithelial cells are exactly like 

 those of the side-walls ; they provide a mucous secretion which is 

 very intensely tinged by carmine, but this reagent has no influence 

 on the cells themselves. Ganglion-cells may be found scattered in 

 the musculature of this region. The so-called subradular organ ap- 

 pears to be sensory ; lying somewhat obliquely below the radula, its 

 hinder portion is circular and is covered by a single layer of very 

 high epithelial cells. More anteriorly the circle is incomplete, the 

 groove dividing the organ into two bilaterally symmetrical halves. 

 In the hinder and upper half of the organ we find spindle-shaped 

 ganglia connected with one another by short commissures. 



After referring to the scantiness of our present information with 

 regard to the gills of the Placophora, the author proceeds to point out 

 that they present a bilateral symmetry, and that the longitudinal 

 fold lying above the so-called branchial groove is not formed by a 

 thickening of the epithelium, but by that of the deeper lying tissue ; 

 anteriorly the longitudinal folds of either side unite above the head ; 

 they also unite posteriorly. In Chiton Imvis there is a special epi- 

 thelial layer below the gills, the cells of which are of considerable 

 height ; these, which have as yet only been observed in the females, 

 appear to be glandular in nature, and it seems to be probable that 

 their secretion stands in some relation to the genital products. Two 

 types may be distinguished in the gills themselves, presenting differ- 

 ences of some importance in the number of the gills, and their relative 

 size. The author is of opinion that the " gills of either side " are not 

 the homologues of the gills of other Gastropods, but that the separate 

 parts are each really distinct gills. A gill, when carefully examined, 

 does not here exhibit the saccular form but is seen to consist of 

 separate transversely disposed plates, connected together superiorly 

 by a longitudinal ridge. The conclusion arrived at is that the 

 Placophora are Poly branchiate Gastropods. No answer can as yet be 

 given to the question whether C. Icevis and Chitonellus with a smaller 

 number of gills are, phylogenetically, older than G. siculus and others 

 with a larger number of gills ; there is no real difference in their 

 structure, there are no signs of any rudimentary gills in C. Icevis, and 

 there does not appear to be any concentration of nervous elements. 

 The key to some of the problems of the structure of the Placophora 

 is doubtless to be found in an examination of Chitonellus. 



Characters of Marionia.* — E. Bergh commences by pointing out 

 that the Tritoniada? form a group which are intermediate between 

 those Nudibranchs with a branched liver (kladohepatic), and those 

 in which it is not branched (holohepatic). The family contains but 

 few forms, all of which are elongated and quadrangular ; in the 

 anterior region of the back there is a delicate semilunar growth, the 

 edges of which are provided with simple or compound digitate pro- 

 cesses. After noting the other special structures of the family, and 

 the characters of its three generic groups, the author comes to the 

 special subject of his paper. 



* MT. Zool. Stat. Neap., Iv. (1883) pp. 303-26 (1 pi.). 



