CHAPTER XX. 



APPENDICES TO THE PLATYHELMIA. 



APPENDIX I. 

 CLASS RHOMBOZOA, v. Ben. 



THE forms included in this group are elongated, symmetrical organ- 

 isms parasitic in the renal sacs of Cephalopoda. The body consists of a 

 single layer of flat cells, which are usually ciliated, enclosing a central or 

 " axial cell," within which the genital cells or " germs " are produced. The 

 ectoderm cells are specialised at one end, to form either a " polar cap " or 

 terminal warts. 



The individuals are of two kinds, producing different embryos one, 

 known as the " nematogen," produces " vermiform embryos " ; the other, 

 the " rhombogen," produces " infusoriform embryos." 



The class Rhombozoa contains two orders. 



ORDER 1. Dicyemida, v. Ben. 1 



Adult forms worm-like and ciliated, with a polar cap formed of eight 

 or of nine cells arranged in two circles. 



Dicyema, KolL, with eight polar cells, contains seven European 

 species (Fig. II.). Dicyemennea, Whitm., 2 with nine polar cells ; this 

 genus includes three species. 



ORDER 2. Heterocyemida, v. Ben. 3 



The ectoderm of the adult is not ciliated ; there is no polar cap, but 

 at the anterior end the ectoderm cells contain refringent bodies and may 

 give rise to four large terminal wart-like papillae (Fig. I.). 



Conocyema, v. Ben., in Octopus vulgaris. Microcyema, v. Ben., without 

 warts, in Sepia ojficinalis. 



Remarks. All the members of the Rhombozoa are parasitic in the 

 renal organs of various species of Cephalopods ; normally, the Dicyemids 

 are attached, as was first observed by Wagener, by means of the polar cap 

 to the renal cells which constitute the so-called "venous appendages." 



1 Ed. v. Beneden, Bull. Acad. Roy. Beige; (2), xli. 1876, pp. 85, 116 ; and xlii. 

 p. 35 ; also in Q. J. Mic. Sci. (N.S.), xvii. 1877, p. 132. 



2 Whitman, Mitth. Zool. Sta. Neapel. iv. 1883, p. 1. 



3 E. v. Beneden, Arch. Biol. iii. 1882, p. 197. 



