14 COMPARATIVE ANATOMY. 



crop are three pairs of minute glands of unknown function 

 called cesophageal glands. 



The membranous partitions extending from the body-wall to 

 the alimentary canal, and dividing the perivisceral cavity into a 

 series of inter-communicating spaces, are well seen in the 

 middle third of the preparation. In the inter-mesenteric spaces 

 of the middle third of the body are to be seen the ' segmental 

 organs/ consisting in each space of a looped internally ciliated 

 tube, communicating by its internal extremity with the peri- 

 visceral cavity, and by its other extremity opening externally. 



Rolleston, p. 119. 



23. The same, dissected to shew its nervous system. 



The integument, divided in the medio-dorsal line, has been 

 pinned out on either side, and the whole of the digestive and 

 generative systems removed. The nervous tract is seen occu- 

 pying the medio-ventral line, and extending from the oral to 

 the anal extremity of the body. The supra-oesophageal mass is 

 seen to consist of two pear-shaped lobes applied together in the 

 median line by their broad ends, their apices pointing outwards. 

 From the outer end of each of these two lobes a large nerve 

 passes forwards to supply the upper lip, and a bristle has been 

 placed underneath them. A commissural cord on each side 

 proceeds downwards from the supra-cesophageal ganglia, en- 

 circling the oesophagus and connecting these ganglia with the 

 ventral nervous cord. 



The anterior portion of the cord is of considerable thickness, 

 and, on close inspection, may be seen to be composed of two 

 distinct strands ; the ganglionic enlargements are, however, with 

 difficulty recognizable. In the middle third of the body the 

 cord becomes much thinner, but assumes a beaded appearance 

 owing to the greater comparative size of the ganglia and the 

 wider interspaces which separate them. In the posterior third 

 of the body it again becomes of considerable size, but the 

 ganglia are less distinctly visible and more closely apposed. 



A pair of nerves is given off from each pair of ganglia, and 

 a single nerve on each side from the segments of the cord 

 interposed between each pair of ganglia. 



24. A Sea-Mouse (Aphrodite aculeata), dissected to shew 

 its digestive organs. 



