NEMATODA. 679 



may be smooth-walled, or covered with small pointed teeth variable in 

 number and size. When terminal it is usually surrounded by small pro- 

 jecting lips formed by the cuticula and sub-cuticula, in number two, three, 

 four, or six, but most commonly three. There are no muscles to these lips, 

 but the hold of the animal is maintained by suction through the oesophagus. 

 In Sclerostomum, and, according to Luckart, also in Dochmius, two long 

 glands, probably poisonous in nature, open into the oral capsule. The 

 digestive tract itself is divisible into three sections : an oesophagus with 

 muscular walls lined by a cuticle which is shed with the cuticle of the 

 body, a mesenteron, and a rectum, the later lined by a cuticle shed in the 

 same manner as that of the oesophagus. The oesophageal walls have an 

 external cuticle, with radial muscles passing from it to the internal lining 

 cuticle. Granular remnants of the original cells with nuclei are to be 

 found here and there between the muscle-fibres, most plentifully in the 

 young. Longitudinal muscle-fibres may also be present. The oesophagus 

 sometimes retains very clearly its cellular origin. This is especially the 

 case in Trichina and Trichocephalus. In Mermis muscular elements are 

 entirely wanting. The cavity of the oesophagus is usually triangular in 

 cross section, and may even be reduced to a three-legged slit. Its hinder- 

 part is dilated into a muscular bulb or gizzard, frequently armed with teeth 

 in HeterakiS) in Oxyuris and its allies. There are often two such bulbs 

 in the Anguillulidae. A solid oesophageal spine is present in Tylenchus 

 Tritici (= Angitillula scandens) and the embryo Mermis ; a hollow tubular 

 spine in the free-living Dorylaimtts. The mesenteron is composed of a 

 single layer of columnar or flattish cells, covered both internally and ex- 

 ternally by chitinoid membranes, which may be resolved into small plates, 

 adapted to the ends of the individual cells, the internal plates being 

 usually perforated by vertical pores. In Trichina spiralis it consists of 

 a single row of cells, one behind the other, perforated by a canal ; so, too, 

 the larval Tylenchus Tritici. It consists in Leptodera, Pelodera, and Pseu- 

 dalius inflexus (?), of two rows of cells alternating from side to side, whilst 

 the genus Strongylus constitutes a transition to the usual structure, in 

 which there are many rows of cells in a transverse section of its walls. 

 A muscular coat is, as a rule, absent ; but in a few instances, e. g. Oxyuris 

 vermicular is , the hinder region is covered by a network of circular fibres ; 

 and there is sometimes a strong sphincter-muscle developed at the junction 

 of the mesenteron with the rectum. These muscles, as well as the mesen- 

 terial or radial bands, which support the mesenteron in situ in some forms, 

 e. g. Eustrongylus, are probably connected, like the muscular coat of the 

 rectum, with the subcuticular fibres. The mesenteron is, properly speaking, 

 cylindrical, or flattened dorso-ventrally, but it becomes deformed by the 

 pressure of the generative organs as they mature. The rectum is usually 

 short, but it is of great proportional length in Trichina ; and it is said not 



