38 WILLIAMSON, ON SHELLS OF CRUSTACEA. 



it impossible to satisfy myself whether or not these present 

 open apertures prior to calcification ; their minuteness and 

 the refraction of light which they occasion rendering the 

 determination of this point one of extreme difficulty. On 

 the peripheral surface of this membrane are numerous reni- 

 form specks having a reticulated aspect. The reticulations 

 resemble cells, and are not unlike the areolae of the areolar 

 layer ; but I believe them to be mere corrugations of the 

 membrane. The specks correspond with equally numerous 

 calcareous projections from the inner surface of the calcified 

 corium. On making a horizontal section of this surface, the 

 projections are seen to be perforated by larger and more 

 sparse tubuli than the contiguous shell, and which display a 

 disposition to areolation similar to what occurs in the corre- 

 sponding parts of the uncalcified corium. In other portions 

 of the same section, patches are often met with, unconnected 

 with the specks just referred to, in which there is also a 

 marked disposition towards areolation, as seen in PI. Ill, fig. 2. 

 These areolae are sometimes hexagonal, but at others so irre- 

 gular as to preclude all idea of a cellular origin. The trans- 

 lucent reticulate lines separating the areola being merely 

 spaces from which the tubuli are absent. 



Throughout the entire shell we find dispersed some cylin- 

 drical bodies which are either fibres or larger tubuli. These 

 penetrate all the layers of the shell. Tubuli similar to these 

 are very numerous in the structureless portions of the pellicle 

 immediately above the flattened summits of the pillars of 

 corium, from which they ascend to the surface of the 

 pellicle. 



Before discussing the moot questions between the observers 

 already quoted, I would direct attention to some other 

 structures calculated to throw light on the debated points. 



On making a vertical section of the tegument of the 

 common shrimp, we find the same number of layers as in 

 the crab, but in a much more attenuated form ; but we 

 have also some new conditions of interest, some of which 

 have already been pointed out by Professor Huxley. 



The areolar layer displays innumerable delicate areolae 

 (fig. 3), like those in the crab, but fainter in outline; both 

 these and the corium appear granulated in the horizontal 

 sections, either as the result of vertical fibrillation or from 

 tubuli. M. Lavalle has denied the existence of tubuli even 

 in the corium of the crab. 



Professor Huxley rejects his conclusions in that instance, 

 but thinks him right in the case of the shrimp, where he 

 believes the granules merely indicate vertical fibrillation ; 



