12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J2 



the occurrence of complicated ramifying roots, so highly developed 

 in some crinoids, is an inherent possibility in the barnacles. 



A combination of the asymmetry of the Verrucidse (inherent also 

 in very many other crustaceans, and especially noticeable in the 

 Paguridas and Bopyridae) carried to its logical conclusion in the com- 

 plete atrophy of one side, with the modifications of the body seen in 

 SphcsrothylacMs or Sarcotaces in a less extreme form, the roots of 

 the Rhizocephala, and a skeleton formed after the manner of the 

 plates in the shell of the Operculata, furnishes all the elements 

 needed for recombination to form the crinoid. It may be well to 

 call attention to the fact that outside the Cirripedia there is no group 

 in which all the morphological peculiarities exhibited in the crinoids 

 coexist — indeed they are not to be found in all the rest of the animal 

 kingdom together. 



In this connection it may be worth while to review the salient 

 features of the more important of the aberrant barnacles. 



The Rhizocephala are exclusively parasitic barnacles, the most 

 degenerate of all parasites; in the adult stage they are distinguished 

 from normal barnacles by the entire absence of all traces of segmen- 

 tation and of appendages, and at all stages they lack an alimentary 

 canal ; every trace of arthropod organization has disappeared. Nearly 

 all of them occur on decapod crustaceans. 



The body has the form of a simple sac, or may be divided into 

 numerous similar sacs, attached by a short peduncle from which 

 root-like processes ramify throughout the body of the host ; these 

 absorptive roots appear to be absent in the aberrant genus Duplorbis. 

 The body proper is completely enveloped by the mantle which usually 

 has a narrow aperture capable of being closed by a sphincter muscle ; 

 in Sylon the opening is double, and in Thompsonia, Clistosaccus and 

 Duplorbis the mantle cavity is completely closed. 



The mantle commonly is attached to the visceral mass by a narrow 

 mesentery near which on either side are the paired (more rarely 

 unpaired) openings of the male and female genital organs. In the 

 different genera the external form varies considerably, and with it 

 the position of the mesentery and of the genital apertures. 



Thompsonia, the most aberrant and highly specialized of all the 

 parasitic barnacles, consists of nothing but a diffuse system of branch- 

 ing and sometimes anastomozing mycelium-like roots continuous 

 throughout the body of the host and all arising from a single original 

 larva ; the peripheral division of the root system passes out into the 

 walking legs, abdominal swimmerets and tail fans and there gives 

 rise to numerous (up to more than 500) small sacs consisting of a 



