324 SUMMARY OF CURRENT RKSF.ARCHES RELATING TO 



descends. An interesting exception is the fact that on moonlight 

 nights, and ('specially at full moon, the rhythm is maintained, and that 

 even although the sky is dark. It is also found that various physical. 

 chemical, or physiological influences are capable of disturbing this 

 rhythm. 



Planaria angulata Muller.* — J. AVilhelmi clears up the confusion 

 in connection with the terminology of Planaria angulata Muller. The 

 name has been given to a Nemertine, a marine Triclad, and a larva 

 whose development has been regarded by several embryologists as in 

 the category of Polyclads. P. angulata Muller must be classed as 

 a Nemertine. The form named by Agassiz as P. angulata has no claim 

 to the title, and further, in consequence of his brief description, is difficult 

 to identify again, although many circumstances point to its being the 

 Bdellurid which lives upon Limulus. Owing to Agassiz' uncertain de- 

 termination there is great doubt as to the value of the observations made 

 by Balfour and by Korscheldt and Heider on the development of 

 P. angulata. 



Structure of Fresh-water Triclads.f — Job. Ude has published a 

 memoir dealing with the anatomy and histology of Planaria gonocephala 

 Dug., Dendrocodum angarense, and D. punctatum. An examination of 

 the characters of Planaria tvgtegrensis, as stated by Sabussows, shows 

 it to be much more probably a variety merely of P. gonocephala. 



Early Development of a Polyclad.J — Frank M. Surface has studied 

 the early stages in the development of Planocera inquilina Wh. The 

 cleavage is strictly spiral in the dextral sequence until a late stage 

 (forty-four cells). Three quartets of ectomeres are given off in alter- 

 nating dexiotropic and Inotropic directions. At the next division a 

 fourth quartet is formed, the cells of which are of very large size and 

 contain most of the yolk. The " macromeres " are very minute cells, 

 which remain at the vegetative pole until the closure of the blastopore. 

 The markedly degenerative character of their nuclei and the small 

 amount of cytoplasm indicate that they degenerate without giving rise 

 to any structure. 



At the stage with forty cells there are formed at the animal pole 

 four small " apical " cells, like those in Annelids and Molluscs. At the 

 forty-four-cell stage the posterior cell of the fourth quartet, ±d, buds a 

 single large cell into the interior of the embryo. Both of these cells, 

 4 d and 4 d 2 , next divide bilaterally. Of these four cells the two upper 

 and inner give rise to a portion of the mesoderm, and possibly a small 

 part of the endoderm. The lower pair of cells, lying on the surface of 

 the embryo, give rise to practically all of the endodermal part of the 

 alimentary canal. Thus the history of this cell, 4 d, shows a remarkable 

 resemblance to its homologue in Molluscs and Annelids. 



The three anterior cells of the fourth quartet, 4«, 4&, and 4^, seem 

 to function only as the bearers of food-yolk, and apparently give rise to 



* Zool. Jabrb., xxvi. (1907) pp. 1-10. 



t Zeitscbr. Wiss. Zool., lxxxix. (1908) pp. 308-70 (3 pis. and 3 figs.). 



X Proc. Acad. Nat. Sci. Philadelphia, 1907, pp. 514-59 (G pis.), 



