248 THE INVERTEBRATA 



section and length ; in the presence of such a cuticle and the absence 

 of circular muscles the peristaltic movements of a worm like 

 Lumbricus are impossible. A cross-section through Rhahditis shows 

 a similar structure to Ascaris, though the muscle cells are much less 

 numerous (only two to each quadrant) : each cell contains a number 

 of contractile fibrils arranged in a different way to those in the Ascaris 

 cell. The body cavity has not been investigated; that of Ascaris has 

 therefore been described above. 



The alimentary canal consists first of all of an ectodermal fore gut 

 lined by cuticle in which the following parts can be distinguished: 

 (i) a mouth, surrounded by papillae, opening into a narrow buccal 

 cavity^ with parallel sides, (2) an oesophagus, with muscular walls and 

 a small number of unicellular glands, forming two swellings, the 

 oesophageal bulbs. The posterior of these (the so-called pharynx) 

 exhibits rhythmical pumping movements, caused by the contraction 

 of the radial muscles which enlarge the cavity of the bulb and open 

 the valve formed by the thickened cuticle. In this way the surrounding 

 fluid is drawn into the oesophagus : no solid particles much larger than 

 bacteria can be admitted through the narrow lumen. When the 

 muscles relax and the cavity disappears the fluid is driven on into the 

 midgut. This is composed of a single layer of cells, which internally are 

 naked but externally have a fine cuticle. These are entirely absorptive 

 in function, gland cells being absent. There are no muscles, but the 

 gut contents are circulated by the locomotory movements of the 

 animal. The hind gut which follows is lined with cuticle and opens at 

 the ventrally situated anus. Near the anus is a sphincter muscle, but 

 there are also dilator muscles running from the hind gut to the body 

 wall, and during the periodic contraction of these the gut contents are 

 evacuated. The alimentary canal of the nematodes as thus seen in 

 action represents a type simplified because the animal usually lives on 

 food which has been split up into easily assimilable substances — in this 

 case by bacterial action, in the case of Ascaris by the ferments of the 

 living host — and this is passed with great rapidity through the 

 alimentary canal by the pumping action of the oesophagus. 



In addition there are easily seen in living Rhabditis the ventral 

 aperture of the excretory canal, not far behind the mouth, and when 

 the animal is compressed under the coverslip the coiled line of the 

 excretory canal ; the only part of the nervous system which can be so 

 seen is the ring round the oesophagus. 



The genital organs are of the type seen is Ascaris but simpler. In the 

 female there are two tubular gonads bent once on themselves, dis- 

 charging by a single genital aperture, situated about half-way between 



^ In some free-living nematodes which are carnivorous (e.g. Mononchus) the 

 buccal cavity is very wide and rotifers and other animals are taken into it. 



