ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 213 



nucleus, where it exists, and the central portion undergo atrophy. It 

 is possible, therefore, that we may be able to distinguish in every 

 cellular element an antagonism or difference in polarity between the 

 central and peripheral portions. These polarities are sexual in 

 character, the central corresponding to the female, and the peripheral 

 to the male. The two polarities have an attraction for one another. 

 Every cell in which they are maintained in equilibrium is a neuter- 

 cell ; it is a complete element, in which nothing is wanting, and 

 which is capable of reproducing itself without any external influence, 

 provided only that it be young enough and have sufficient nutriment. 

 Every cell in which the equilibrium is destroyed becomes a sexual 

 cell, with a predominant polarity ; and this result may be due to a 

 modification in its nutrition and development. We see therefore that 

 every cell which loses its central element becomes a male, and every 

 one in which the central portion is predominant becomes a female 

 cell. Further evidence in support of these propositions is promised. 



Pilidium-Stage of a Nemertine.* — Mr. E. B. Wilson describes 

 the Pilidium stage of a nemertine. It is helmet-shaped, with the 

 convex side more elevated than usual, and crowned by a small 

 flagellum. The anterior margin of the bell is produced into four 

 short arms, behind which is a deep sinus, followed by two arms on 

 each side, the anterior largest of all. The bell is transparent, its 

 walls and lobes very contractile, and its outer and inner surfaces 

 covered with cilia, which are longest on the margins of the lateral 

 lobes. The young nemertines are developed in a folded position, 

 within the lower and posterior part of the larval envelope, and are 

 distinctly segmented posteriorly. 



Tseniadse Parasitic in Birds.t — H. Krabbe has notes on 42 

 species, of which 16 are new. A few synonyms are indicated, 

 and a list, arranged according to the systematic classification of birds, 

 is given of the forms herein described. 



Dicyemid8e,+ — C. 0. Whitman closely criticizes (and denies) the 

 accuracy of E. van Beneden's statements that each Cephalopod has a 

 single species of JDicyema, and that the species found in closely allied 

 Cephalopoda are much more nearly related than those found in species 

 belonging to the different families. He sets himself to show that 

 " one species of Bicyema occurs in at least two different species of 

 Cephalopod, while another is found in at least three different Cepha- 

 lopods " ; that in Eledone moschata and Sepia officinalis there are in 

 each two species of Bicyema; and that of these two (one in each 

 species) differ less from one another than from the species with which 

 they are respectively associated. He proposes to distinguish the 

 Dicyemids by the number of " polar cells," or cells in the head, and 

 to speak of that in which there are eight cells as Bicyema, and that 

 in which there are nine as Bicyemmennea. It will be remembered 

 that van Beneden made four genera. Systematic descriptions of 



* Amer. Natural., xvii. (1883) pp. 94-5. 



t Vid. Selsk. Skrift., i. (1882) pp. 349-66 (2 pis.). 



X MT. Zool. Stat. Neapel, iv. (1882) pp. 1-89 (5 pis.). 



