556 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.55. 



and sides of the cephalothorax and may be small and more or less 

 spherical in shape, or produced into immense foliaceous affairs, 

 larger than all the rest of the body and themselves covered with 

 secondary processes, simple or branched. 



This condition occurs in the genus Sphyrion, but to interpret the 

 secondary processes as modified legs (pattes transformes), as was 

 done by Thor (1900, p. 280) and Quidor (1912, p. XLI), is mani- 

 festly out of the question. That would render it necessary for us to 

 assume the fusion of the entire thorax with the head, which the 

 breaks in the longitudinal musculature show to be impossible. 

 These soft processes have thin walls and are filled with spongy chitin- 

 ogen tissue, v/hich is continuous with that filling the cavity of the 

 cephalothorax proper. 



The horns are of narrow diameter and fully chitinized; they are 

 found at the junction of the cephalothorax with the neck, and are 

 developments of the latter, as is shown in the new genus Periplexis, 

 where they are developed over a considerable portion of the anterior 

 neck, and also in the genus Retelula^ where the entire neck is often 

 covered with chitin knobs. They may be either simple or branched, 

 the branching sometimes becoming very profuse, as in the new 

 species, Rehclula cornuta. 



In Periplexis and Rehelula the cephalothorax is an elongated cyl- 

 inder, with all the soft processes at the anterior end, which is the 

 true head. The remaining thoracic portion may be either smooth, as 

 in most species of Rcbelula., or transversely wrinlded, as in Peri- 

 plexis and Rehelula cornuta. 



This wrinkling, hoAvever, may be more or less the result of preser- 

 vation. 



The neck, with the exception of the modifications just noted, is 

 usually smooth and of the same diameter throughout, but may be 

 enlarged a little posteriorly {SpMjrion) or wrinkled where it joins 

 the trunk {Paeon) . Since these outgrowths of the head and neck are 

 modifications which begin only after the female has burrowed into 

 the tissues of the host, and which increase in complexity with subse- 

 quent growth in size, it is evident that they possess but little specific 

 value. At all events two species of the same genus can not be satisfac- 

 torily established upon these characters alone, as has been attempted 

 in the genus Sphyrion. If young females can be compared, differences 

 in the position, arrangement and form of the outgrowths Avill have 

 far more value than in older and more mature specimens. But even 

 then the chief basis of differentiation must be the form and structure 

 of the appendages. Because of the scarcity of the younger stages and 

 because the adult female looses most of her distinguishing appendages, 

 we must of necessity turn to the male for diagnostic characters. 



