A Contribut. to the Embiyol., Life-history, and Classificat. of the Dicyemids. 15 



bers at any one time ; and it does not seem at ali improbable that a 

 single individuai would suffice to stock a wbole renai chamber. 



As will be seen from Piate 1 , tbe head of D. moscliatum exhibits 

 a remarkable diversity of forms , as the products of two ever varying 

 factors, (1) the degree of obliquity, and (2) the relative size and shape 

 of the component cells. So wide is the range of variability, that I was 

 for some time doubtful as to the number of species represented. A com- 

 parison of a very large niimber of individuai forms, and a study of the 

 various stages of development, bave not brought to light any characters 

 by which one or more of these forms could be specifically separated 

 from the rest. So far as I bave been able to ascertain, the whole num- 

 ber of ectoderm cells — barring cases plainly abnormal — is invariably 

 24, both in the Rhombogens and the Nematogens ; and the extremes of 

 obliquity are fully bridged over by intermediate forms. The calottes of 

 the young are always orthotropal, and essentially alike in the form, 

 size, and disposition of the cells. The dififereuces among the adult 

 forms must therefore be of an accideutal rather than a specific nature. 

 That the calotte of a single species, within the same species of Cephal- 

 opod, should display such a Protean variety of form, is, to say the 

 least, a very interesting fact, and one well calculated to show how ex- 

 tremely unreliable must be the form and relative size of the polar cells 

 in the determination of species. It must be admitted also that this fact 

 gives some grounds to suspect that ali Dicyemids of the octamerous 

 type consti tute but a single species , the so-called species being only 

 varieties, owing ali their distinctive differences to the unlike conditions 

 of life to which they are exposed in their respective hosts. The assump- 

 tion, however, involved in this view, that these conditions are suffi- 

 ciently unlike to induce Constant differences in size and in the number of 

 cells , as well as characteristic varieties of form , is certainly not sup- 

 ported by the fact that the same species of Dicyema may live , as will 

 presently appear, in two or more distinct species of Cephalopod, without 

 the loss or additiou of a single celi, and without any noticeable altera- 

 tion in length, or in the size, shape, and disposition of the cells. The 

 opinion formely entertained by Kölliker, that ali Dicyemids beloug 

 to one and the same species . to which the name Dicyema paradoxum 

 was given , is stili more decidedly disproved by the fact that two en- 

 tirely distinct species, representing different genera, as I hold, may live 

 side by side in the same species of Cephalopod. Van Beneden's fail- 

 ure to recognize this fact in the case of E. moschata must not be taken 

 as an evidence that the differences between the two species are not con- 



