170 



days of germ-layer cult, suggest theoretical conceptions which may be 

 inapplicable to the Dicyemidae. 



In the ectoderm five zones, each consisting of several cells, follow 

 one another in regular sequence from the anterior to the posterior end. 

 The odd-numbered zones, i. e. the first, third and fifth are constant 

 in the number of their cells in all known Dicyemidae. Of the even- 

 numbered zones, the second exhibits a generic, the fourth a specific 

 variation in the number of its constituent elements. Thus in all known 

 Dicyemidae in the first zone there are four cells known as the pro- 

 polars. The second, or metapolar zone may consist of four five, or 

 six cells. The pro- and metapolar zones taken together form the "ca- 

 lotte", which may, therefore, be octamerous (in Dicyema), ennea- 

 merous (in Prof. Whitman's genus Dicyemennea) or decamerous 

 (in a new genus which I shall call Dicyemodeca). The three genera are 

 represented each by a single species in the Octopus of the Pacific 

 coast [Dicyema coluber n. sp., Dicyemennea Whitmanii n. sp., and 

 Dicyemodeca sceptrum n. sp.). The calotte of the European Dicyemids 

 may be either radially (orthotropal) or bilaterally symmetrical 

 (plag io trop al) . All the American species known to me have ortho- 

 tropal calottes. The third zone always consists of two cells, the par a- 

 polars. The same is true of the fifth zone, or uropolars as they 

 may be called. Between the parapolars and uropolars extends the 

 longest and most variable region, consisting of from ten to eighteen 

 cells in different species. These cells, which I shall call diapolars 

 envelop the axial cell as a spiral band, a single revolution of which 

 may consist of either two or three of these cells according to the spe- 

 cies. As the animal grows older certain diapolars, the uropolars and 

 in some cases also the parapolars may become filled with refractive 

 granules and bulge out beyond the remaining ectoderm cells. In this 

 condition they are called verruciform cells. Their function is un- 

 known. 



The calotte, which is more densely ciliated than the remaining 

 ectoderm, attaches the Dicyemid to the surface of the venous append- 

 ages of its Cephalopod host, while the body of the parasite floats out 

 in the renal liquid. When the parasites are abundant they form a 

 pilose or velvety coating to the venous appendages. When detached 

 they move about in the renal liquid, bending their protoplasmic bodies 

 easily and gracefully. Dicyema coluber, which attains a length of 

 2 — 5 mm, is able to coil itself up like a watch-spring. 



The reproductive elements make their appearance and develop 

 within the axial cell, all known Dicyemidae being viviparous. Besides 

 the large nucleus the axial cell of the fully formed Dicyemid, even 



