Chap, iv.] SUCTORIAL HABITS. 179 



on their contraction the cavity of the oesophagus is 

 greatly increased, and a vacuum is thereby formed; 

 here, then, the tube acts as a sucking-pump. The 

 great care needed in making any generalisations in 

 comparative physiology is well spoken to by the 

 curious fact that among the parasitic Nematoids a 

 mouthless condition obtains only in the free-living 

 stages ; thus Gordius has a mouth when parasitic, 

 but when it leads a free life the mouth is lost, and the 

 worm is dependent on the store accumulated in its 

 earlier stage ; in its ally, Mermis, the peri-oesophageal 

 muscles are lost in the free stage of existence, though 

 the mouth remains. In the leech the sucking action 

 is effected in essentially the same way as in the 

 Nematoid. In the scorpion, where the mouth is ex- 

 cessively minute, the pharynx is pear-shaped, and has 

 attached to its wall transversely set muscles, which, 

 on contraction, increase the extent of the cavity, and 

 so cause a vacuum which results in an up-flow of the 

 fluids of the prey which it has stung to death. A 

 large number of Araclmida have a distinct sucking 

 apparatus. 



The changes induced by parasitic habits on the 

 conformation of external parts of the body are, as 

 may be supposed, most striking in the Arthropoda, 

 Among the Entomostraca a very instructive series of 

 gradations may be made out. The gnathites of 

 Cyclops are in Caligus, which is a temporary parasite 

 on fishes, enclosed in a tube formed by the fore-and- 

 hmd-lips ; the anterior pair, or mandibles, are alone 

 well developed, and form piercing processes. In 

 Corycpeus the suctorial tube is not developed. In both 

 of these there are swimming feet. In Lernsea, which 

 may be found on the gills of the cod, to which the 

 adult females are permanently attached, the swimming 

 feet are small. In Achtheres, which is found on the 

 perch, these feet are wanting, and a pair of gnathites 



