INVERTEBRATE ZOOLOGY. 83 



which unite and open by a median ventral pore. The nervous system consists 

 of an oesophageal ring and a pair of longitudinal cords, dorsal and ventral, 

 which are borne by ridges similar to those which contain the nephridia, but 

 smaller. Both nephridia and nervous cords are best seen in a cross-section 

 [58]. Circulatory and respiratory organs fail. 



58. Cross-section of Ascaris (preferably lumbricoides). Look first for 

 four projections of the body-wall, 90° apart and corresponding to the four 

 ridges just mentioned. These will serve to orient the section. The two lat- 

 eral ridges are much the largest, and contain each a cross-section of a delicate 

 tube, the nephridiiun. The ridges for the nervous cords support rather than 

 contain them, and thus appear somewhat cup-shaped. T)earing a whitish re- 

 fractive solid substance, the nervous cord. The hody-n-all consists principally 

 of an outer transparent ciiticula and a thick mass of irregular muscle-cells, 

 some of which encroach upon the coelom. A fibrous layer between the two 

 and directly underlying the cuticula may l)e looked upon as a sort of hypo- 

 dermis. originally cellular, but showing in the adult only a few scattered 

 nticlei. The alimentary canal occupies typically a median position and con- 

 sists of a single layer of somewhat irregular epithelial cells, with a thin inner 

 cuticula. A muscular layer fails. The remaining tubules, lying in the coe- 

 lom, l)elong to the reproductive system and present different aspects in size 

 and cellular formations according to sex, period of development, and regions 

 cut by the plane of the section. In sections of the mature uterus, eggs may 

 fi'equently be met with, jjrovided with a transparent .shell showing a double 

 contour. The embryoes develop to a certain stage in the Ixxly of the parent, 

 and may be seen in the early segmentation stages, or in the blastula or gas- 

 trula form. 



59. Additional studies of Nenuitodes. There are several other forms of 

 considerable general importance, which should be investigated as occasion 

 offers. 



a. " Vinegar-eel " {A)ignillida aceti). Hold a vinegar-cruet up to the light 

 and look for exceedingly minute worms, frequently so numerous as to give 

 the impression of some optical peculiarity in the liquid which causes it to 

 shimmer. These are minute nematodes, feeding upon the fungus which 

 converts the cider into vinegar, and called the "mother."" All specimens of 

 vinegar do not contain these nematodes, but if a specimen containing a bit of 

 the fungus be taken, one is almost certain to find them. Pipette a little of 

 such vinegar into a watch crystal and observe with the dissecting microscope. 

 Single nematodes may be transferred to a slide, covered and examined with 

 higher powers. 



