ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 22 L 



peripheral margin of the layer of pigmeDt-cells passes into the retina ; the 

 want of observation of this may be ascribed to the great delicacy of the 

 cells in this region. Put generally, we find that the retina and pigment- 

 layer form a closed optic vesicle, which, however, differs essentially from 

 that of other molluscs in that the sensitive region is turned away from the 

 light. This great difference appears to be easily explicable by the dif- 

 ference between the lenses of the two groups of molluscs. In molluscs 

 other than Pecten the lens is a secretion-product formed in the interior of 

 the optic vesicle, where it permanently or temporarily remains ; in Pecten, 

 and also in the Vertebrata, the cellular lens is formed outside the optic 

 vesicle, and so the outer wall of the optic vesicle is turned towards it. 



North Sea Mollusca.* — In the second part of the report on the 

 Mollusca collected by the Norwegian North Sea Expedition Herr H. Friele 

 describes the Pleurotomid^, Cancellaria, and Brachiopoda ; there were a 

 number of species of JBela. Asbjoernsenia is a new genus, which is placed 

 before Montacuia and after Philine. 



MoUuscoida. 



a. Tunicata. 



The Salpa-chain-t — Mr. W. E. Brooks compares the development of 

 the chain of Salpa to that of Pyrosoma. 



He first describes briefly the development of the young Pyrosoma, and 

 gives a detailed description of that of Salpa, the arrangement of the young, 

 Salpse on the stolon, and the changes in position during growth. As in 

 Pyrosoma, the young Salpse are not produced by budding from the walls of 

 the stolon, but arise by conversion of the segments of this structure into 

 the bodies of the new organisms. But whereas in Pyrosoma there are only 

 three or four young ones in successive stages of development, in Saljpa there 

 are many successive se^s, each consisting of 60 to 100 individuals at the same 

 stage. The stolon arises from the haemal surface of the solitary Saljpa, 

 and consists of an outer wall of ectoderm and an inner endodermal tube, 

 which communicates with the branchial sac of the parent, and arises from 

 the floor of the ventral groove. 



At first the young Salpa-ch&in is bilaterally symmetrical, but in the 

 mature chain the individuals are placed in a rather complicated fashion, 

 which arises from crowding and pressure. In reality there is a single 

 series, each placed dorsum to venter, with the neural surface towards the 

 base of the stolon, and the right sides of all on the right side of the stolon, 

 so that the middle plane of symmetry coincides with the middle plane of 

 the body of each Salpa. 



By a rotation caused by crowding the complicated arrangement of the 

 mature chain is brought about. 



Mr. Brooks finds that horizontal sections are the only means of ascer- 

 taining the true relations, as each succcessive Salpa will be cut in a 

 different plane. 



The arrangement of the Salpse to the stolon is given in detail. 



Synascidians new to the French Coast. | — M. A. Giard gives a notice 

 of Diazona Jiehridica and Pistaplia rosea, which appear to be new to the 

 coast of France. M. Giard thinks that Diazona is a composite Clavelinid, 

 and he points out that it approaches the simple Ascidians by the fact that 



* Den Norske Nordhavs Expedition, 1876-8. xvi. Zoologi. Mollusca, ii. 4to., Chris- 

 tiania, 1886, 44 pp. and 5 pis. (Norwegian and English in parallel columns.) 

 t Stud. Biol. Lab. Johns-Hopkins Univ., iii, (1886) pp. 451-73 (2 pis.). 

 X Oomptes Rendus, ciii, (1886) pp. 755-7. 



