2l8 THE INVERTEBRATA 



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 evacu- 

 ated. The alimentary canal of the nematodes as thus seen in action 

 represents a type simplified because the animal 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 rhabdites 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 in 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 

 the head and the tail. The ovary is a short syncytial tube, the nuclei 

 becoming larger and larger and the centre of more definite and larger 

 aggregations of cytoplasm and yolk nearer the uterus. Finally, there 

 is a single ovum discharged at a time into the oviduct ; as soon as this 

 happens another ripens in its place. To reach the uterus the egg has 

 first to pass through a portion of the oviduct {receptaculum seminis) 

 filled with the amoeboid spermatozoa of the male. Fertilization takes 

 place, a shell is formed and at the same time maturation proceeds. 



