190 SUMMARY OF CURREI^T RESEARCHES RELATING TO 



INVERTEBRATA. 



MoUusca. 



7. Gastropoda. 



Radula of Mitridse.— A. H. Cooke {Proc. Zool Soc, 1919, 405-22, 

 18 figs.). The rbachidian tooth exhibits wide differences in structure, 

 ranging from the lozenge-shaped 8- to 9-cusped form in 3Iitra to 

 the unicuspid triangular form of the variegata group. These divergences 

 in the structure of the rhachidian are accompanied bj a general similarity 

 of plan in the laterals, subject however to a progressive modification in 

 their form. The laterals, in fact, exhibit every symptom of regress 

 towards a gradual degradation. Perhaps there is a chie here to the 

 genesis of the familiar bicuspid or unicuspid lateral of many of the 

 Rhachiglossa. It seems within the bounds of possibility that the coales- 

 cing, or gradual disappearance, of the cusps, in a multicuspid lateral, 

 produced in more cases than that of the Mitridge a lateral with one or 

 two large cusps instead of many small ones. J. A. T. 



Sensory Reactions of Chromodoris zebra. — W. J. Crozier and 

 Leslie B. Arey {Journ. Exper. Zool., 1919, 29, 261-310, 8 figs.). Ex- 

 periments on mechanical, photic, thermal and chemical excitation of this 

 Nudibranch. There are differentiated receptors mediating reactions to 

 tactile, chemical and shading stimulation, to the constant intensity of 

 light, and perhaps to heat. Locally, the responses of the general integu- 

 ment and all the outgrowths depend upon locally contained, peripheral, 

 non-synaptic networks, which are polarized in the gill-plumes and 

 probably in other projecting parts. Reactions involving parts distant 

 from the site of activation depend upon central, ganglionic transmission. 

 The central nervous system is essentially synaptic. The Nudibranch is 

 positively phototropic, the chief receptive organs probably being the 

 eyes, but the gill-plumes are also sensitive, expanding in light, retracting 

 when shaded. When sexually mature, the animal is negatively geotropic. 

 It is negatively rheotropic to strong Avater currents, the directive organs 

 being the " rhinophores." Chemotropic reactions to body secretions of 

 other individuals lead to conjugation, "olfactory" and "gustatory" 

 stimulation being concerned. The locomotion is primarily muscuhir, not 

 ciliary. The outer lateral margins of the foot are active. The foot is 

 positively stereotropic, and when removed from a surface folds together 

 laterally. This suits creeping on narrow blades of eel-grass. The stereo- 

 tropism of the anterior end of the foot is responsible for righting 

 behaviour. There is no apparent statolithic control for dorso- ventral 

 body orientation. J. A. T. 



Sensory Responses of Chiton.— Leslie B. Arey and W. J. 

 Crozier {Journ. Exper. Zool., 1919, 29, 157-260, 14 figs.). An account 

 of the general natural history of Chiton inhere nlatus, its movements and 

 reactions, and its responses to mechanical, thermal, photic, and chemical 

 excitation. The sensory conditions are unexpectedly complex ; the 



