AND EMBRYOLOGY OP LIMULUS. 13 



conspicuous where the preparations have been stained with haematoxyhn ; ^ the cell 

 walls are difficult to distinguish with a one-fifth objective. The pavement epitheliuiii fills 

 the spaces between the folds of the oesophagus and crop (or fore gut), and is succeeded 

 by a single layer of columnar epithelium, which looks like a delicate rufiie, edging 

 the folds, and lying between the pavement epithelium and the chitinous lining of 

 the canal. The chitinous layer is very finely laminated, the laminae being parallel 

 for the most part to the indentations and projections of the folds and the teeth of 

 the fore-gut, showing plainly that it is secreted by the layer of columnar epithelium. 

 Cross sections of the larva, after hatching, through the fore-, mid-, and hind-gut, when 

 the appendages and internal organs have assumed their definite shape, show that 

 the intestine then consists of only two layers, the muscular, which is comparatively 

 thin, and the layer of columnar epithelium (plate 5, figs. 7, 7a), which rests directly 

 upon the muscular layer, and consists of long cells projecting irregularly into the 

 cavity of the canal. It would thus appear that the thick layer of pavement epithelium 

 and of chitine is not developed throughout the intestine, until some time after hatching. 

 Indeed, it is known that the larva lives for a long time, even months, after hatching, 

 before it takes much, if any, food. 



Returning to the oesophagus; it is seen to be lined with a pale yellowish chitinous 

 layer gathered into about eight large deep folds. Plate 5, fig. 5, illustrates the structure 

 of two of these folds and part of the adjoining ones. The muscular fibres are not 

 represented. The cells 4, 4a, of the pavement epithelium {pe) are round or oval, with a 

 large, distinct, dark nucleus; their walls are difficult to define. The projecting lobes con- 

 sist of columnar epithelium, with large nuclei, much more distinct than in the pavement 

 epithelium ; the basal half of the cells are dark, being filled compactly with granular mat- 

 ter enclosing the nuclei, while on the outer half the cells are transparent ; plate 5, fig. 3, 

 3a, represents these cells enlarged. The lobes are hollow, leaving a clear space, as 

 shown in figure 5 ; the lobes are unequal in form and size, those figured being situated 

 near the posterior end of the oesophagus. The columnar epithelium is succeeded by the 

 chitinous layer (cA), which is finely laminated, the laminae corresponding to the direction 

 of the lobes. 



The crop or proventriculus consists of three parts ; in the most anterior division the 

 chitinous folds, continuous with those of the oesophagus, are large and irregular and extend 

 vertically upwards, until they bend backwards suddenly at right angles to form the rows 

 of thick, solid teeth lining the second or middle and larger part of the crop. These teeth 

 are arranged in five sets of rows, each set or series consisting of three rows, and two series 

 of two rows, the two latter sets situated on the under or ventral side of the stomach, and 

 arranged on each side of the three-rowed series. The teeth in each row are nearly uniform 

 in size, are transverse, being flattened antero-posteriorly. In the three-rowed series, 

 especially on the ventral side, the teeth of the middle of the three, rows are larger than 

 those of the row on each side. There are about 225 well marked teeth in this division 

 of the crop, those at either end of the rows being small and sometimes double. 



1 1 am much indebted to Dr. C. B. Johnson, of Providence, preparations of the oesophagus, crop, and intestines, stained 

 for kindly cutting, staining, and mounting some excellent both with haematoxylin and carmine. 



