ARTHilOPODA. 27 



organs. An opereular process, attached in front, and free poste- 

 riorly, hangs like a flap over this aperture. This may probably 

 be regarded as the sternal element of this segment. The 

 sternal elements of the two maxillary segments of the 'cephalo- 

 thorax' are aborted, but in the two succeeding segments the 

 sterna, though small, are still sufficiently obvious. 



The segments and appendages hitherto described belong to 

 the cephalo- thorax: the five succeeding ones to the abdominal 

 region. The abdominal segments are distinct from each other, 

 and, with the doubtful exception of the first, carry no append- 

 ages. Each consists of a broad tergal and sternal arc, united 

 laterally by non-chitinous membrane. The sternal element of 

 each segment is divided into a median division, 'sternum/ and 

 two lateral divisions, *episterna,'by two well-marked grooves, one 

 on each side of the median line. On each ' episternum,' with 

 the exception of that of the first pair, is a small oblique slit, 

 the external opening of the pulmonary sacs. In addition to the 

 absence of pulmonary apertures, the first abdominal segment 

 further differs from the rest in the small size of the sternum, and 

 in the presence of a pair of pectiniform appendages. Each ap- 

 pendage presents indications of being composed of several seg- 

 ments, and carries on its hinder border a row of close-set filaments. 



The six segments which succeed the abdomen are the 

 post-abdominal segments. With the exception of the first, 

 which resembles the abdominal segments in its general 

 shape, each segment is more or less cylindriform, completely 

 chitinized, convex above and concave below, and marked with 

 longitudinal serrated ridges. Attached to the last post-abdomi- 

 nal segment is a conical, somewhat tumid segment, which 

 carries a strong curved spine upon its distal extremity, and may 

 represent the 'telson' of the Crustacea. On the ventral surface, 

 and in the interspace of the last two segments, is the anus. 



46. A Scorpion (Scorpio ), dissected to shew its 



digestive tract. 



The alimentary canal is seen to be a straight tube, 

 destitute of convolutions, and passing directly from mouth 

 to anus. The liver is very large, occupying four-fifths of the 

 thoracic cavity, and from it numerous ducts pass to the stomach. 

 The anus is situated on the under surface of the last segment 



