930 RECORD OF CURRENT RESEARCHES RELATING TO 



Striated Muscles in the Monomyarian Acephalous MoUusca.* — 

 Although the existence of such muscles was affirmed by Eeichert as 

 early as 1842, and by subsequent authors, in certain Cephalopoda, 

 Acephala, and Gasteropoda, on the ground of a transverse striation 

 which was noticed, yet M. Blanchard has determined these facts to 

 have been wrongly interpreted : such striation in the cases advanced 

 was either due to the contracted state of the fibres, or to a special 

 arrangement of the intracellular or interfibrillar granular matter of the 

 muscles. 



On the other hand, their existence in certain Mollusca is now 

 established by his own investigations, viz, in the Pectinidce alone. 



In a Pecten (P.jacoheus, e, g.) the adductor muscle is compound; 

 a smaller, white, shining division is separated from the rest of the 

 mass by a membrane proceeding from the sheath, and is formed of 

 smooth fibres ; tbe larger mass, which consists of striated fibres, is 

 dull grey in colour. 



This tissue, like the muscle of the wing of Htjdropliilus, is formed of 

 a number of very delicate parallel fibrillfe not united by sarcolemma, but 

 they are not interspersed with granular matter, as in the muscle of the 

 insects' wing. Each fibril extends from one valve to the other. Besides 

 a coarse transverse striation, an extremely fine one is_ distinguishable 

 by a power of 500 to 600 diameters ; here the " thick disks " seen in the 

 wing muscle of HydropMlus alternate with " clear spaces," which here 

 too are crossed by " thin disks " ; by treatment with chromic acid or 

 with dilute alcohol the " clear spaces " are also seen in the thick disks, 

 in some fibrils here also the thick and thin disks colour strongly with 

 carmine, and especially with ha^matoxylin. Polarized light exhibits 

 the " disks " as doubly, and the clear spaces as singly, refractive, 

 as in Vertebrata. The muscle of Pecten is however distinguished 

 more evidently from that of E ijdropMlus by the occurrence on each 

 fibril of a large elongated nucleus which projects from the surface, 

 colours strongly with carmine, and contains granular protoplasm. The 

 mean diameter of the fibril is -01 mm., varying to '02 mm.; the 

 mean length of the nucleus is from -01 to -012 mm., its breadth from 

 •004 to -005. 



If the adductor muscle of Pecten is homologous, as M. Lacaze- 

 Duthiers has declared, with the posterior adductor of the Dimyaria, 

 one would expect to find striated fibres in this muscle also ; but they 

 are absent from this and all other parts of Mytilus edidis, Anodonta, 

 and Unio hitherto investigated. The compound nature of the muscle 

 in the former case points rather to the conclusion arrived at by 

 Gegenbaur, that it consists of the two originally distinct adductors. 



As an object for histological study this muscle is preferable to that 

 of the HydropMlus wing, as it is easy to fix it either in the relaxed 

 or contracted state ; the fibres are larger and more easily isolated, and 

 the granular substance which interferes with the preparation in the 

 latter case is here absent. Although the tissue has been vainly sought 

 for in the Gasteropods and in various races of the oyster, yet in the 

 latter form the adductor presents very ditierent characters in its two 

 halves, so that it gives encouragement to further investigation. 

 * ■ Kev. lutciiiiit. Soi.,' v. (1880) p. 356. 



