ALIMENTARY APPARATUS. 365 



having the last joint expanded and ciliated, as in some, but not all, of 

 the aquatic tribes ; for running (cursorii), where it is long and slender ; 

 for weaving (textorii), in which case the ultimate segment is provided 

 with very short and much-curved hooks, and the antepenultimate with 

 four elongated stiif bristles longer than the foot ; and lastly, such as are 

 formed for a parasitic life, or carunculated (carunculati), in which, su- 

 peradded to the hooks, is a caruncle or broad membrane wherewith the 

 creature fixes itself to a smooth surface, something in the same way as 

 the sucker of a Leech. 



(939.) The mouth is composed of two moveable pieces called the 

 mandibles, beneath which is a broad plate (labium), which is either flat 

 or folded laterally so as to form a kind of gutter, and, moreover, is fur- 

 nished on each side with a rudimentary palpus. The mandibles are 

 generally free, but in some cases are united together and conjoined with 

 the labial piece, so as to form a short tube or proboscis, near the end of 

 which may be perceived a pair of moveable tooth-like structures, adapted 

 to pierce the substances whence these suctorial races obtain their liquid 

 food. When the mandibles remain entirely free and moveable, they ex- 

 hibit, as was pointed out by M. Duges*, three principal modifications 

 in their structure: 1st, they are forcipated (fig. 184), like those of 

 Scorpions; secondly, they may be terminated by a single moveable 

 fang (fig. 186, c), as is the case in Spiders; and lastly, they may be 

 composed of two long styles which are capable of alternate movements 

 backwards and forwards, whereby they can perforate foreign substances, 

 much in the same manner as the saw of the Tenihredo among insects. 

 The first of these forms are never provided with any poison-apparatus, 

 and are only adapted to tear and pull to pieces alimentary substances ; 

 but in the second form poison-glands are superadded to the curved fangs, 

 which, as in the proper Arachnidans, thus become formidable weapons. 



(940.) The arrangement of the digestive apparatus in the Acaridans 

 is one of the most interesting points in the economy of these creatures. 

 Behind the mouth, M. Dujardinf was able to detect, in Trombidium and 

 Limnochares, a cylindrical pharynx, with distinct parietes, into which 

 are implanted numerous muscular fibres calculated to assist in the 

 operation of suction by dilating the pharyngeal cavity ; but posterior to 

 this no traces are perceptible of either oesophagus, stomach, or intestine, 

 so that, apparently, the juices of organized bodies, which constitute the 

 sole food of these creatures, must be lodged in lacunary spaces, destitute 

 of any proper walls, in the middle of a brown parenchymatous mass, 

 which probably performs the functions of the liver. The Iacuna3, into 

 which nourishment is thus received, must necessarily be prolonged 

 amongst the tissues and in the interspaces between the muscular fasci- 

 culi throughout the entire body, thus replacing altogether the circu- 

 lating fluid ; and even when living specimens of such genera (Derma- 

 * Ann. des Sci. Nat. 1834. t Id. 1845, t. iii. p. 14. 



