AND EMBRYOLOGY OF LIMULUS. 39 



instance of the metamorphosis and cephalization of the nervous system, which is carried 

 on internally, though the other organs and outer body-form remain unchanged. 



Development of the digestive canal. Unfortunately the mode of formation of the primi- 

 tive digestive cavity or archenteron was not observed, as eggs showing the formation of 

 a gastrula could not be obtained. From this early period until after the larva has 

 hatched the entire canal remains unorganized, the entire body-cavity being filled between 

 the heart and nervous tract with yolk granules. 



The earliest stage when the enteric canal was observed at all was after the different 

 parts — oesophagus, crop, stomach, intestines, and cloaca or rectum — had assumed their 

 definitive shape. Plate 4, fig. 2, illustrates a section of the larva before its first moult, 

 through the head. The space around the heart and digestive canal and over the nervous 

 cord is filled with a very loose connective tissue ; the cells, which are nucleated, spindle- 

 shaped or triangular, being scattered, and forming a very open net-work of cells. In after- 

 life the cells multiply, becoming very numerous and round or oval in form. This con- 

 nective tissue extends throughout the entire body-cavity, the ovarian or testicular tubes 

 ramifying throughout the mass, as well as the liver tubules. 



The section at plate 5, fig. 8, passes through the oesophagus and the crop. The former 

 (figs. 10, 11, enlarged) is apparently filled with a few large epithelial cells, which represent 

 the folds of the lining of the oesophagus. The walls of the proventriculus are very thick ; 

 the lumen or passage is lined with the alternating larger and smaller folds of spherical 

 epithelial cells, and with a thin semi-chitinous layer ; the muscular layer, representing the 

 endoderm buUt up around the originally invaginated ectodermal layer forming the fore gut 

 or protenteron (plate 5, figs. 7, 7a), shows the epithelium of the intestine, the cells being 

 very irregular in size and length. 



Origin of the liver. Plate 4, fig. 7, represents a section through the middle of the 

 cephalothorax, passing through the intestine and one of the pairs of biliary ducts. The 

 ducts are seen to open directly into the stomach, the duct being large, and at first there 

 is a primary liver-tube, which bends downward at quite an angle before passing to the 

 outer edge of the carapace. There are thus four primary biliary tubes, these in after 

 life subdividing and ramifying throughout the body-cavity to an indefinite extent. The 

 tubes are clear, transparent, with dark granules. 



Development of the ovary. The same section represented in plate 4, fig. 7, also passes 

 two bodies, one on the outer side of and just below the heart, on each side of the mid-o-ut. 

 These are the rudimentary ovaries. One section (fig. 8) shows the ovarian follicles 

 attached to the walls of the gland, and, in fig. 8a, the ovarian eggs are just beginning to 

 form, constituting a mass apparently free from the walls of the ovarian tubules. 



Structure of the testes and development of the spermatozoa. In our first memoir on Limu- 

 lus we figured the spermatozoa ; since then Professor Lankester has also described them. 



The argument that Limulus is not a Crustacean because the spermatozoa have tails is 

 somewhat vitiated by the fact that those of the barnacles have exceedingly long, well 



