ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 147 



those of certain Heteropoda, and on the retina in Cephalopoda. The 

 compound eyes of Area the author considers to be without parallel 

 among other Mollusca, though he is of opinion that the remarkable 

 receptive cells — the retinophorae of Patten — may occur among such 

 Mollusca as are known to be sensitive to light, although " eyes" have 

 not been described. The eyes have a striking resemblance to those of 

 the worm Brancliiomma. In certain species of Lima there occur pig- 

 mented epithelial pits, which resemble the eyes of Patella, and are to be 

 regarded as " eyes " — that is, as organs for the reception of light. In 

 the much-investigated eyes of Pecten the author figures some remark- 

 able structures in the cells of the lens. These cells contain a very 

 distinct central corpuscle, with a system of radii, extending to the 

 periphery of the cell. Their function the author believes to be to increase 

 the contractility of the lens. In addition there is an arrangement of 

 fibrils which appear to alter the shape of the lens, and so accommodate 

 the eye to different distances. Many new details are also given as to 

 the structure of the different parts of the eye. In regard to the Hetero- 

 poda, it is found that there is considerable uniformity in the structure of 

 the eyes. The most notable variation is in regard to the presence or 

 absence of one or more gaps in the pigmentation of the choroid. These 

 pigment-free " windows " are always associated with the development of 

 accessory receptive-cells, which receive the light entering through the 

 gaps. The structure of the receptive-cells suggests to the author the 

 idea that the rhabdom of the Arthropod eye may have arisen from the 

 fusion of the striated margins of receptive-cells such as occur in many 

 Mollusca. 



Cruciform Muscle of Tellinacea.* — H. von Ihering finds that all 

 members of the Tellinacea have at the base of the siphons in the con- 

 nected ventral parts of the margins of the mantle a singular muscle, 

 formed by two crossing muscles which are inserted in the valves in the 

 region of the angle formed by the mantle-impression and the sinus, or 

 between it and the borders of the shell. One of the two branches per- 

 forates the other, both being united into a cross-shaped muscle, which 

 functionally may 6erve as a secondary adductor. Evidently this cruci- 

 form muscle is a special development of fibres of the mantle-edge which 

 have been isolated only secondarily from these margins, and this, accord- 

 ing to the author, was the origin of the adductor-muscles as well. 



Arthropoda. 



a. Insecta. 



Morphology of Insect's Head.j — M. Charles Janet has made a 

 most careful study of this difficult subject, with especial reference to 

 Myrmica rubra and Yespa crabro. The criteria as to number of somites 

 are to be found in the development, in the suture-lines, in the articu- 

 lating membranes of the chitinous pieces, in the nervous system, and 

 especially, perhaps, in the musculature. It is not difficult to distinguish 

 six divisions — the ucron, the antennary segment, the post-antennary (in- 



* Proc. Acad. Nat. Sci. Philadelphia, 1900, pp. 480-1 (2 figs.), 

 t ' Essai but la constitution morphologique de la tete de l'lnsecte,' Paris, 1899, 

 74 pp., 7 pla. See Zool. Centralbl., vii. (1900) pp. 800-4. 



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