180 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



unite to form a single sucker, or disc of attachment. 

 As we pass from Cyclops to Lernsea or Achtheres, the 

 form of the body becomes less and less obviously seg- 

 mented, and more and more bizarre in appearance. In 

 Argulus, the common parasite of the stickleback and 

 other fishes, the changes are still more marked ; the 

 suctorial tube is large, and has within a pair of finely 

 toothed mandibles and style -shaped maxillae ; in 

 front of the mouth is a pointed spinous tube, contain- 

 ing the ducts of what are supposed to be poison 

 glands ; the anterior pair of maxillipeds (Glaus) form 

 large sucking discs on either side of the mouth. 



Among the higher Crustacea the parasitic forms 

 belong to the group of the Isopoda ; the Bopyridse 

 have a sucking proboscis, and their mandible is with- 

 out a palp ; some, like Entoniscus and Cryptoniscus, 

 are ordinarily lernseoid in form, when adult ; one 

 species of the latter, which is parasitic on a Sacculina, 

 which is parasitic on a Pagurus, has so peculiar a 

 form as to have received the specific name of plana- 

 roides. 



Among the Arachnida, many of the mites 

 (Acarina) are parasitic, and the bases of the two an- 

 terior pairs of appendages form a sucking proboscis, 

 as in Demodex, which dwells in the hair follicles of 

 various mammals ; the females of itch-mites are able 

 to bore under the skin ; in the blood-sucking ticks the 

 proboscis is provided with a number of hooks. In 

 Pentastomum, which has two hosts, and is endo-para- 

 sitic in both of them, the only signs of appendages to 

 the body are the two hooks on either side of the 

 mouth. 



Parasitism is very rare among Mollusca ; Mont- 

 acuta lives among the spines of Spatangus, Stylifer on 

 sea-urchins, among corals, or in starfishes, but neither 

 of these are specially modified. Entoconcha, parasitic 

 in Holothurians, is merely known as an ovigerous sac. 



