380 Bibliographical Notices. 



towards the older parts. The concentric lines of growth occurring 

 on the outer surface were totally absent on the inner surface, which 

 is explained by supposing the apposition at the margin to stop 

 for some time, the formation of new layers at the whole under 

 surface at the same time continuing. 



The chief result of the investigation of the body-wall (with its 

 pallial lobes) on surface-views and sections was the demonstration m 

 of the non-existence of the lacunary system described by Hancock. 

 Under the simple epiblastic epithelial layer was found a homogene- 

 ous intercellular substance, containing a reticular network of multi- 

 polar cells — a u mesenchyinatic " layer. These cells have probably 

 been mistaken by Hancock for a system of lacunar}' spaces. 



At the side of the coelomic cavity the body-wall shows a layer of 

 flat epithelial cells. 



The so-called tendinous portions of the muscles were found to be 

 specially developed portions of the mesenchymatic layer under the 

 real insertions of the muscular fibres. This was most obviously 

 shown in the occlusor muscles of Wdldheimia, where the tendons 

 arc united, to a considerable extent, with the body- wall, and where, 

 in transverse sections, not the slightest difference or limit between 

 them is to be found. 



The same origin must be attributed to the peduncle, which, con- 

 trary to that of Lingula, consistsof a solid mass of mesenchymatic tissue 



containing many fibres. Such fibres .are also found in the margin of 

 the mantle and the free or inner walls of its sinuses ; they serve for 

 support, and were believed to be muscular by Hancock. 



The epiblastic epithelium is everywhere a unicellular layer, ex- 

 cept on a small area under the mouth, where the nervous system is 

 in immediate contact with it. No cellular laver was found at the 

 outer surface of the mantle-lobes under the shells ; it only showed a 

 reticular design, corresponding to the bases of the shell-prisms, and 

 therefore most regular at the border of the mantle. 



The nervous system is described by the author as a nervous collar 

 with a large infra- and a small supraoesophagcal ganglion. In the 

 description of the first he agrees with Hancock ; but in that of the 

 latter he quite differs from him, inasmuch as a median ganglionic 

 mass is described passing on each side, without any sharp demarca- 

 tion, into a broad nerve, running along the brachial groove and in- 

 nervating the brachial fold. The commissures uniting this supra- 

 oesophageal ganglion with the nervous centrum under the mouth 

 are very thin and supported by two membranous inward prolonga- 

 tions of the body-wall, while the two centra themselves lay in the 

 mesenchymatic substance of the body-wall itself, the supraoesopha- 

 geal immediately under the ectodermal epithelium, the infraoeso- 

 phageal separated from it, except along its upper border, by a layer 

 of homogeneous tissue. From the infracesophageal ganglion also 

 an arm -nerve was found to originate, which, running parallel to the 

 first-mentioned supraoesophageal nerve, innervates the bases of the 

 cirri. Both these arm-nerves were surrounded by a network of 

 many large cells that communicated with them everywhere. 



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