A Contribut. to the Embryol., Life-history, and Classificat. of the Dicyeuiids. 7 



according to Brock (24) in the closest genetic relationship , are 

 fouud two species of Dicyema differing from eacli other as widely as 

 either difìfers from D. moschatuni of E. moschata. 



These facts make it perfectly clear that 1 could not pursue the 

 method of grouping adopted by Van Beneden, nor accept the generic 

 divisions to whìch his erroneous views had gi\ en rise. 



Ali the Dicyemids at present known may be easily and conveniently 

 divided iuto two groups or genera , distinguishable by the number of 

 cells in the head — polar cells of van Beneden. The first genus, to 

 which the name Dicyema^ originatìug with Kölliker, will be applied, 

 includes seveu species, ali with eight polar cells. The secoud 

 genus embraces three species, ali characterized by niue polar cells, 

 and may therefore be designated, Dicyemennea'^. 



Explanatory Remarks. 



Passing over histological details , which appear to bave been very 

 accurately given by Van Beneden , I shall bere point oiit those char- 

 acters by which the genera and species at present known may be most 

 readily recognized ; and , in so doing , shall adhere to the terminology 

 employed by Van Beneden, so far as this meets my purpose. To avoid 

 repetition and }»eriphrases, a few explanatory remarks and defiuitions of 

 the more frequently recurring terms are bere introduced. 



For systematic purposes, the cephalic enlargeraent is by far the 

 most important part of the animai, as it furuishes the greater number 

 of the generic and specific characters. The eight, or ulne cells compos- 

 ing this portion , are always in two sets , and the first set invariably 

 contains four cells. These two sets of polar cells form together the po- 

 lar calotte (»coifie polaire», V. Ben.). The cells of the two sets may be 

 designated according to their relative position , which is invariable in 

 young individuals and persistent in the majority of species, pro -polar 

 and meta- polar. In some cases, the pro-polar cells become, in the 

 adults, centrai, and the meta-polar, peripheral. The bilateral sym- 

 metry of the calotte , first recognized by Van Beneden, will necessitate 

 further the use of the terms dorsal, ventral, and lateral. The ex- 

 tremity of the body adjoining the calotte, is universally formed of two 



1 Dicyema and évvéa , uine. I should add that there is some doiibt about the 

 position of D. Miilleri, as Ci.aparède failed to determine the number of polar 

 cells. 



