26 SUMMARY OF OUTiRENT RESEARCHES RELATING TO 



Thecosomata thus corresponds to the sum of the four ganglia of tho 

 visceral commissure. 



In general, (a) tho pleural ganglia are paired in Gymnosornata as in 

 all molluscs where they are present ; (b) tho buccal appendages of 

 Gymnosornata aro innervated by cerebral ganglia, and cannot therefore 

 be compared with Cephalopod arms ; (c) the Pteropods aro thus separated 

 from Cephalojiods. The asymmetry of their visceral commissure 

 separates them from all molluscs with symmetrical visceral commissure. 

 They approach tho Gasteropjods, and especially, as Spengel noted, the 

 Euthyneura. 



'Challenger' Pteropoda (Gymnosornata).* — Dr. P. Pelseneer has 

 published tho first part of his report on the Pteropoda collected by 

 H.M.S. ' Challenger,' which has become a critical account of all known 

 genera and species. The adult Gymnosornata are characterized by the 

 absence of a mantle-skirt, pallial cavity, and shell ; by the presence of a 

 well-developed head, bearing two pairs of tentacles, of which the two 

 posterior bear rudimentary eyes ; by two fins of which the anterior 

 edges are not joined together backwards, above the mouth ; and by the 

 anus being situated at the right side of the body. They are carnivorous, 

 and often feed on their thecosomatous allies. Eleven species were 

 collected by the ' Challenger,' four of which are new ; all the known 

 twenty-one forms are discussed in the systematic portion of this memoir. 



5. Lamellibranchiata. 



Photogenic Property of Pholas dactylus.| — M. E. Dubois has made 

 a series of experiments which show that tho photogenic property of 

 Pholas dactylus is independent of any organ, and is a chemical 

 phenomenon. From the luminous parts of the animal the author has 

 succeeded in extracting two substances, the contact of which, in the 

 presence of water, determines the appearance of the light. One of them 

 was obtained in the crystalline state, and possesses the special optic 

 properties which give to photogenic tissues their opalescence. It is 

 soluble in water, and hardly soluble in alcohol ; it may be called luci- 

 ferine. The other body is an active albuminoid of the class of soluble 

 ferments, and may be called luciferase. These two substances are 

 necessary to, and sufficient for the production in vitro of the phenomena 

 of animal luminosity, improperly called phosphorescence. The results 

 here obtained confirm and generalize those attained to by the author 

 after his study of the luminous Elaterida?. 



• Molluscoida. 



a. Tunicata. 



Central Nervous System.^ — M. F. Lahille has studied the develop- 

 ment of the central nervous system in a largo number of Tunicate 

 embryos, and comes to the following conclusions. The typical central 

 system consists of a median tube of epiblastic origin, with bilateral 

 symmetry, and with numerous ganglionic masses. If the principal 

 masses are considered as forming so many ganglia, the following may be 

 distinguished : (1) the anterior (tactile) ; (2) the sensory (ocular and 



* Reports of the Voyage of H.M.S. ' Challenger,' Iviii. (1887) 72 pp. and 3 pis. 

 t Comptes Bendus, cv. (1887) pp. 690-2. J Ibid., pp. 957-60. 



