540 SUMMARY OF CURRENT RESEARCHES RELATING TO 



are again optic grooves; but on the inner side of the cuticula, which 

 closes the grooves, a lens has developed (e.g. Asterina gtbbosa, Aster ias 

 glacialis, Echinasler sepositus). These different cases are described and 

 compared with optic organs in other animals. 



Polarity of Oocyte, Ovum, and Larva of Strongylocentrotus 

 lividus.* — Prof. Th. Boveri finds that the polarity of the larva may 

 be securely traced back to the oocyte of the first order, and with the 

 strongest probability to the oogonia of the germinal epithelium. The 

 basal and the free surface in the latter correspond to the animal and 

 vegetative pole of the oocytes. In the oocytes liberated from the 

 ovarian wall the animal pole is marked by the canal of the gelatinous 

 envelope and by the eccentricity of the germinal vesicle. The two 

 cleavages of the oocytes always take place at the animal pole ; the 

 polar bodies emerge through the above-mentioned canal. In the ovum 

 which results the pigment previously uniformly distributed in the peri- 

 phery is concentrated in an annular zone, at right angles to the axis of 

 the egg, and belonging to the vegetative half. The segmentation is 

 continued in strict relation to this polarity. Especially is the differ- 

 entiation of the larval organs determined by the three zones on the 

 ovum, marked by the pigment-ring. The vegetative unpigmented cap 

 forms the primary mesenchyme and the larval skeleton ; the pigmented 

 zone forms the gut and its derivatives, the unpigmented animal half 

 forms the ectoderm and its differentiations. 



North American Holothuroids.j — H. L. Clark contributes to the 

 useful series of synopses of North-American Invertebrates a brief ac- 

 count of the Holothuroids, reducible to not more than forty-seven 

 species, a list of the specific names no longer even tolerable, an arti- 

 ficial key of the genera (Synapta, Ghirodota, Myriotrochus, Eupygus, 

 Trochostoma, Caudina, Psoitis, Thyonepisolus, Thyone, Cucumaria, Mulleria, 

 Stichopus, and Holothuria), and a key to the species under each genus. 



Ccelentera. 



Variations of a Newly-Arisen Species of Leptomedusoid4 — A. 

 G. Mayer has made a careful study of the variations of Pseudoclylia 

 pentata, a Leptomedusoid of the family Eucopidae, which he discovered 

 in large numbers at the Tortugas, Florida. It differs from all other 

 Hydromedusas in that it normally possesses five radial canals, five lips, 

 and five gonads, all 72° apart, instead of four of these various organs, 

 90° apart as in other Eucopidas. In the structure of its tentacles, oto- 

 cysts, gonads, and manubrium, the general shape of its bell, and the 

 arrangement of tentacles and otocysts, Pseudoclylia pentata is very 

 closely similar to Epenthesis folleata McCrady, except that the former 

 is pentamerous while the latter is tetramerous. The two medusae are, 

 however, different in colour, so that one can readily distinguish quadratic 

 aberrations of P. pentata from normal specimens of E. folleata, and, 

 vice versa, pentamerous individuals of E. folleata from normal P. pentata. 

 But the two are so similar that it seems safe to conclude that P. pentata 



* Zool. Jahrb., xiv. (1901) pp. 630-53 (3 pis.). 



t Amer. Nat., xxxv. (1901) pp. 479-96 (27 figs.). 



% Sci. Bull. Brooklyn Inst., i. (1901) pp. 1-27 (2 pis.). 



