12 A. S. PACKARD, JR., ON THE ANATOMY 



Nebalia, and that the posterior region is truly an alidomen, the spine of Limulus 

 being simi^ly the last body-segment, or ninth abdominal arthromere, as the history 

 of the endiryonic development of this segment proves. It then follows that the 

 abdominal resjjiratory feet are, for example, homologues of the broad respiratory 

 abdominal ajjpendages of Isopoda. The view of Mr. Woodward, that what we regard 

 as abdomen represents in part the thorax, or the opinion of Owen and Huxley tliat the 

 s^Dine represents the abdomen, and that what we call the abdomen is the thorax, in 

 jjart at least, is, it seems to us, not based on sound induction. 



Histology of the Internal Organs of the Adult Limulus. 



Histology of the digestive system. The general form of the digestive canal is 

 seen in plate 3, fig. 1. The large mouth-opening is situated between the third to 

 fifth pairs of limits. The oesophagus is very long, and directed very obliquely forward 

 and upwards from the mouth, entering the large crop or proventriculus at an angle 

 to the general course of the latter, which is full and large, projecting anteriorly 

 over the end of the oesophagus. It curves over backwards, growing smaller posteriorly, 

 projecting above slightly over the beginning of the stomach or mid-gut. What we call 

 the crop, is the " cardia " of Van der Iloeven, and the "cardiac end of the stomach" 

 of Owen and A. Milne-Edwards. Communication with the chyle-stomach is effected 

 by the large internal projection in the form of a truncated cone (plate 3, fig. 1, cone), 

 by which the food, when partially digested, is strained, and passes from the proventriculus 

 into the true stomach. The latter, externally, seems to form the beginning of the 

 intestine, and extends from the base of the proventricular projection backwards as 

 far as the first pair of biliary ducts ; its histology is quite diflerent from that of the 

 proventriculus and its posterior conical process. 



The beginning of the intestine is indicated externally by a slight contraction just 

 before the origin of the anterior of the two pairs of biliary ducts. These are placed 

 far apart by a distance nearly equal to twice the thickness of the intestine. The 

 hind gut is divided into the intestine and rectum. The intestine is straight, and 

 of uniform thickness as far as the beginning of the rectum, which is swollen, owing to the 

 large rectal folds within. 



On laying open the digestive canal of specimens collected in the winter, it is found 

 to be filled with a jelly-like substance, which on examination proves to be the lining 

 of the canal, which has been molted, and has undergone partial digestion. 



Examining the inner walls of the digestive canal, and studying its histology, we 

 find that there are three fundamental layers composing the canal, extending from the 

 mouth to the vent. There are, beginning on tlie outside, (1) the muscular layer, 

 (2) the mucous or ejnthelial lajer, and (3) the chitinous layer. The muscular 

 layer is made up of longitudinal muscles, the fibres striated, with scattered small bundles 

 of transverse striated fibres, some of these isolated from the outer layer of longitudinal 

 muscles and passing through the epithelial tissue. 



The second or epithelial layer is thick, composed of pavement epithelium, arranged in 

 fibrous masses or bundles, somewhat like muscular tissue. The nuclei are large and 



