ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 495 



phone group. Eacli group touctes four neiglibouring ones, and every 

 peripheral cell is in contact with one of its like. Where the four 

 groups meet there is a separate intermediate cell, which serves as the 

 centre for fresh formations. 



The initial cell grows rapidly, its protoplasm becomes distinguish- 

 able from that of the neighbouring cells, and the nucleus gets a well- 

 marked contour. A little later indications of its pigmented character 

 are to be seen, and then become more and more abundant. The peri- 

 pheral cells meanwhile divide and give rise to the 16 to 22 cells which 

 are the hasal cells of the adult. Each intermediate cell divides 

 actively and gives rise to an intermediate group, which grows with the 

 general growth of the integumentary surface, and so separates the 

 chromatophoric groups. Some of the cells give rise to fresh chroma- 

 tophoric groups, while others form the connective tissue of the layer 

 in which the chromatophores are placed, while others, lastly, become 

 fresh intermediate cells. The second set form linear series and give 

 rise to reticula of various forms ; the nuclei separate one from another, 

 and the course of formation of the fibres may be well seen in embryos 

 treated with Kleinenberg's solution. The history of the development 

 of the new intermediate cells explains how it is that, in the adult, we 

 find chromatophores in different stages of development ; and, on the 

 whole, the intermediate cells may be regarded as constituting the 

 fundamental connective tissue of the layer, and the radial fibres which 

 converge towards the chromatophoric groups ; while, moreover, they 

 allow of the increase in extent of the layer, by giving rise to areas 

 in which the same phenomena are observed as in the initial em- 

 bryonic formations. 



Organization of Chitons.* — B. Haller has continued his investi- 

 gations, and here commences with an account of the structure of the 

 buccal muscles, to which interest has attached since Boll's discovery 

 of larger cells lying on the muscles ; these latter themselves may be 

 regarded as consisting of an internal contractile substance, arranged 

 in longitudinal fibres, which, however, cannot be isolated or separated 

 from one another ; their surfaces are really fused so that the longi- 

 tudinal fibrillation only is apparent. This internal contractile sub- 

 stance is surrounded by a layer of protoplasm which contains a 

 number of nuclei and has obviously a nutrient function ; the larger 

 cells would seem to be portions of this outer layer, which becomes 

 compressed whenever a wave of contraction passes along the muscle. 



The buccal cavity seems to be the chief seat of the sensory organs 

 of the Chitons, otocysts and eyes being absent ; among the epithelial 

 cells of the lips there may be found the brush-cells of Flemming, of 

 essentially the same form as in the Pulmonata, where they were first 

 discovered. An unpaired elevation on the floor of the cavity seems to 

 be gustatory in function, and consists of sensory and supporting cells ; 

 the goblet-cells which are so common in the buccal cavity of the 

 Prosobranchiate Gastropoda are here completely wanting. There 

 is a pair of feebly developed buccal glands, which are mere diverticula 



* Arbeit. Zool. Inst. Wien, v. (1883) pp. 29-60 (3 pis.). 



