ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 63 



Polyclads of the Pacific Expedition.* — Dr. Marianne Plehn de- 

 scribes the Polyclads collected by Prof. Schauinsland. From Tasmania 

 came a form for which a new genus is erected, called Microcelis on 

 account of the small eyes. The species, M. schauinslandi, is described 

 in detail ; it is characterised by the extraordinary number of minute 

 eyes in the anterior region. 



Development of Nemertea.j — Mr. W. E. Coe has studied the de- 

 velopment of the pilidium in Cerebratulus leidyi, C. marginatus, and 

 Micrura cceca. In the last named there is little yolk, and the eggs are 

 transparent. They " furnish an almost ideal example of the regular 

 spiral type of cleavage," and the blastomeres are almost equal in size. 

 Gastrulation occurs by invagination ; but before it begins the larval 

 mesoderm originates from the endoderm. From a comparison of the 

 three species it appears that the mesoderm arises both from the division 

 of a posterior pole-cell, and from the pushing in of certain endoderm- 

 cells into the segmentation-cavity. By the end of the first day the 

 dorsal surface of the embryo becomes flattened, and the down-growth 

 of ectoderm forms the side-lobes of the pilidium. The apical plate 

 with its long flagella arises at the extreme upper end of the embryo 

 where flattening does not occur. The pilidium swims freely near the 

 surface of the water with the flagella directed upwards. It feeds on 

 small pelagic larvae, diatoms, &c The enteron is divided into two 

 regions, an anterior buccal cavity and a posterior blind intestine, the 

 two being separated by amoeboid cell-processes which form a valve. 

 The two regions are formed of histologically distinct elements, and 

 probably the anterior is ectodermic and the posterior endodermic, but 

 the two originate by an " almost continuous invagination." 



In the apical plate, as already shown by Biitschli, originally sepa- 

 rate flagella fuse to form a single thickened flagellum. As in Annelid 

 trochophores, the apical plate is connected by muscles with the digestive 

 tract and the sides of the body. The pilidia studied showed no trace 

 of the complex nervous system described by Salensky. The muscles 

 originate from the mesenchymatous cells, which multiply rapidly during 

 gastrulation. Owing to the extreme transparency of the larvae, the 

 gradual transformation of amoeboid mesenchyme cells into definite 

 muscle-fibres can be watched in the living larva. The fibres ultimately 

 anastomose with one another to constitute a complicated system of 

 interdependent muscles. 



The transformation of the pilidium into the young Nemertean was 

 not observed. 



Breeding-times of Nemerteans.J — Mr. W. K. Coe notes that the 

 eggs of some New England Nemerteans can be obtained in abundance 

 at almost any season of the year, and that they afford most useful 

 objects for embryological and cytological investigation. He states the 

 breeding-time for 17 species. Nearly all the common species become 

 sexually mature on the southern coast of New England during the 

 summer months. Only one lays its eggs in mid-winter, and only two 

 in the very early spring. 



* Zool. Jahrb., xii. (1899) pp. 448-52 (2 figs.). 



t Trans. Connecticut Acad., x. (1899) pp. 235-62 (5 pis.). 



t Science, ix. (1899) pp. 167-9. 



