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to explain the obscure by the more obscure. Prof. Whitman, too, 

 maintains that there are two kinds of females, but not in the sense of 

 van Beneden. He calls certain females monogenic, or primary 

 nematogens, on the supposition that they produce only vermifomi 

 young. Others, he believes, are first rhombogens and later in life ne- 

 matogens. These he calls diphygenic. 



My own study of Dicyema coluber leads me to adopt a very differ- 

 ent and a much simpler view. I began by preserving in formalin all 

 the Dicyemidae taken from a hundred Octopus punctatus of different 

 sizes, and by noting the character of the embryos in each case. The 

 results of this work are given greatly condensed in the following 

 table 2; 



56 Octopus 1 — 3 cm long. Contained only nematogens, the 

 number of the parasites increasing with the age of the host. 



6 Octopus 3,5 cm long. Nearly all nematogens, a few young 

 rhombogens (i. e. with young infusorigens) . 



8 Octopus 4 — 5 cm long. Nematogens and rhombogens, the latter 

 often preponderating, but always young. 



6 Octopus b^h — 6cm long. Mostly rhombogens with more advanced 

 infusoriform embryos; very few nematogens. 



21 Octopus 7 — 12 cm long. Rhombogens except in very rare cases 

 -when a few nematogens were found; no nematogens in the larger spe- 

 cimens of Octopus (10 — 12 cm). 



3 Octopus 22 — 25 cm. All rhombogens with very old infusoriform 

 embryos and old infusorigens; axial cell often empty for long distances 

 as if exhausted of its embryos. 



From these observations I inferred, first, that the same 

 Dicyema is both nematogen and rhombogen at different 

 periods ofits life, and second, that it is first a nematogen 

 and then a rhombogen, i. e. it develops in the reverse order 

 of Prof. Whitman's sequence. 



I next confined my attention to the Dicyemidae taken from Ce- 

 phalopods 4 — 5 cm long, the stage in which the transformation of the 

 nematogen into the rhombogen should occur if my inference was cor- 

 rect. In searching through a great amount of material I succeeded 

 in finding some ten or twelve Dicyema which certainly 



2 The measurements of the Cephalopods were taken from the posterior end of 

 the body to the middle of a line joining the anterior edges of the eyes, so that the 

 length of the arms is not included. The method of weighing adopted by Prof. AVhit- 

 man would have been preferable to measuring these contractile mollusks, but no sa- 

 tisfactory balances were at hand when I was working at San Diego, Cala, where 

 most of my material was collected. 



