196 



LOWER INVERTEBRATES. 



develop into a parasite, which, were its history not known, v/ould not be supjjosed to 

 have any connection with the species to which it really belongs. 



The Polystomese include a great variety of strangely-shaped parasites, the major- 

 ity of ■v^hich infest fishes. Especially remarkable are the marine forms, many of which 

 have been described by the elder Van Beneden, and illustrated by a series of beauti- 

 ful plates, which record the bizarre outlines and colors of many species. Little, how- 

 ever, is yet known concerning the history of most of these trematods. 



The following paragraphs, for which I am indebted to my able friend. Dr. C O. 

 Whitman, refer to a group of worms, the Dicyemidae, which he has studied more pro- 

 foundly than any otlier zoologist. Although their systematic position is doubtful, it 

 seems to me most probable that they are larval trematods. 



The DiCYEMiDJE are parasitic worms inhabiting the renal organ of the cuttle-fish, 

 the poulp, and other cephalopods. Only ten species have thus far been described, and 

 these, with one exception, belong to the fauna of the Mediterranean. These creatures 



are attached to the renal organ by the head, the body 

 floating free in the fluid that fills the sack enclosing 

 the renal organ. The different species vary in length 

 from 1 to 7 mm., and all have the same habits and 

 life history. The entire renal organ is often beset 

 by these animals, which, to the naked eye, appears 

 as slioi't, white, undulating hairs. They have no 

 mouth, no stomach, no muscle, no nerve, nor organ 

 of any kind. The entire animal is made up of a few 

 cells varying, according to the species, from twenty 

 to thirty. There is one long axial cell (Fig. 179, 

 en) stretching through the entire length of the para- 

 site ; the remaining cells form an envelope (ec) for 

 the axial cell. The cells of the epithelial envelope 

 are arranged in a single layer, and clothed with vib- 

 ratile cilia. At the anterior fixed end, a certain 

 number of these cells, which are elsewhere elongated 

 and thin, are short and thick, and more closely cili- 

 ated, thus forming what may be called a head. The 

 number, arrangement, and shape of the head cells, 

 furnish, according to Whitman, the chief generic 

 and specific distinctions. Seven species have each 

 eight head cells, disposed in two sets, four propolar 

 {ap), and four metapolar {nt.p), and thus form a generic group, to which the name 

 Dicyema has been given (Fig. 179, A). The three remaining species have each nine 

 head cells (Fig. 179, B), four propolars {ap), and five metapolars {mp), and have there- 

 fore received the generic name Dicyemennea. 



The structural simplicity of these animals is most probably due to degeneration 

 resulting from their parasitic mode of life. "When we find an animal in the form of 

 a simple sack, filled with reproductive elements, secured by position against enemies, 

 supplied with food in abundance, and combining parasitism with immobility, we have 

 strong reasons for believing that the simplicity of its structure is more or less the 

 result of the luxurious conditions of life which it enjoys, even if its development fur- 

 nishes no positive evidence of degeneration." Physiologically speaking, the axial ceU. 



Fig. 179. — A, Dicyema. B, THcyemennea; 

 ap, propolar cells; e, embryo; en, axial 

 cell; ec, outer cell; mp, metapolar cells; 

 «, nucleus. 



