ZOOLOGY AND BOTANY, MIOKOSCOPY, ETC. 935 



no certain evidence that these pores allow of the direct entrance of water 

 into the blood. The blood-corpuscles may be large or small, and the 

 two kinds differ somewhat in the structure of their nuclei. 



There can be no doubt that a genus of MoUusca which has no heart, 

 no internal renal orifices, and no gills, must occupy an isolated place in 

 the system. Dr. Plate believes that Lacaze-Duthiers was wrong in 

 ascribing to it a greater affinity with the Lamellibranchs than with the 

 Gastropods. The single shell, the possession of jaws and a radula, and 

 the whole arrangement of the nervous system, especially the existence 

 of two pleural ganglia, are signs of Gastropod affinities. The so-called 

 oral cone ouglit to be regarded as a head, the eyes and appendages of 

 which have been lost, as in Chiton and some other forms. On the other 

 hand, the hypothesis that Dentalium is the ancestor of the Cephalopoda 

 cannot be supported ; theie is no greater resemblance than there is 

 between a Cephalopod and any Gastropod which has retained its bilateral 

 symmetry. There are great difficulties in homologizing the tentacles of 

 Dentalium with the arms of the Cephalopoda, for, inter alia, the former 

 are at the base and the latter at the tip of the head, and there are con- 

 siderable differences in the supply of their nerves. 



5. Lamellibrancliiata. 



Structure of Muscles of Lamellibranchiata.* — M. E. Blanchard 

 has studied the structure of muscular tissue in various lamellibranch 

 molluscs. He finds that the constituent element is a fibre-cell 1 to 2 mm. 

 long, and 4 to 38 /x wide ; the nucleus is superficial and marginal, and 

 has no enveloping membrane. This fibre is fundamentally structureless 

 or, at most, is infiltrated with fine granulations, but it often presents a 

 longitudinal striation. This last varies much in distinctness, for at first 

 it is only feebly differentiated. In this case the surface of the cell often 

 presents various ornamentations ; this structure is inconstant, and the 

 author is unable to suggest an explanation of it ; all that can be certainly 

 said is that it would be a great mistake to compare it to true transverse 

 striation, or, as M. Fol has done, to regard it as due to the spiral rolling 

 of the fibrils. In some further points M. Blanchard is in disagreement 

 with M. Fol, with whose observations he deals more fully in another 

 communication."!* 



Formation of Byssus4 — Dr. L. Eeichel communicates the results of 

 his investigations on the formation of the byssus in Lamellibranchs. 

 These results are based chiefly on the observation of living specimens of 

 Dreyssena polymorpJia, and on microscopic examination of sections of the 

 foot, the byssus-cavity, and the byssus of the same animal. Preparations 

 of Mytilus edulis and Pinna were used for comparison. The bulk of the 

 memoir is occupied with a discussion of the attaching function of the 

 byssus, and with the details of its development. Dr. Eeichel affirms 

 that the byssus arises as a cuticular formation, and that it is not a per- 

 manent structure which lasts the animal its lifetime, but that, like the 

 skin of Arthropods, it can be thrown off, and gradually replaced. The 

 throwing off of the byssus is accompanied by a degeneration of the byssus 

 cavity. The walls of partition are reduced, and the cavity becomes a 

 simple groove, but they are formed anew when a new byssus arises. 



* Bull. Soc. Zool. France, xiii. (1888) pp. 74-81. f Tom. cit., pp. 49-55. 



X Zool. Beitr. (Schneider), ii. (1888) pp, 107-32 (1 p].). 



