770 RECORD OF CURRENT RESEARCHES RELATING TO 



ment with Ihering, wlio would separate Gasteropteron, Philine, and 

 Scaphander from the Bullidea ; the French anatomist would, however, 

 retain Woodward's family, and would form in it two subdivisions, in 

 one of which O aster opter on is the only genus ; these two subdivisions 

 may be thus characterized. In the first, the paraj^odia are largely 

 developed, a small nautiloid shell is contained within the mantle, and 

 the oesophageal collar is made up of a pair of cerebral ganglia, of a 

 pair of pedal, and of six visceral ganglia ; the last being arranged by 

 three to the right, and three to the left. 



In the second division the parapodia are always rudimentary, the 

 shell is always very distinct, is never nautiliform, and may be well 

 developed and external ; there are only three visceral ganglia in the 

 oesophageal collar, and of these, two are placed to the right, and one 

 to the left. Here, too, we find that the genital nerve always arises 

 from the larger of the two left visceral ganglia, while in Gasteropteron 

 it arises directly from the commissure without the intermediation of 

 any ganglionic enlargement ; while the branchial nerve, which, in 

 Gasteropteron always arises from the right visceral centres, may in 

 them be derived from the right visceral ganglion, from the right half 

 of the commissure, or from a ganglion placed in the middle of this 

 connecting cord. 



Development of Teredo.* — Dr. Hatschek has extended his obser- 

 vations in development to the Lamellibranchiate Mollusca. 



The youngest ovarian ova are pyriform in shape, and are attached 

 by their stalk to the wall of the ovary ; the germinal vesicle is excentric ; 

 the fertilized ova and the embryos are found within the gills of the 

 mother, where in numerous individuals it is often possible to see three 

 different stages ; the older being in the more anterior region. It is 

 soon possible to observe in an unsegmented ovum a clear animal and 

 a darker vegetative pole ; after the first segmentation we 'have two 

 unequal spheres, the smaller or more anterior of which is not so dark 

 as the other, in consequence of the less close packing of the yolk- 

 granules. The author is of opinion that in all Bilateria a bilateral 

 symmetry is to be made out in the ovum, just as in all Metazoa there 

 is a polar differentiation of the same cell. Observations on the process 

 of segmentation show that the ectoderm is formed from the clearer 

 cells, while the unpaired larger segmentation-sphere goes to form the 

 mesoderm and endoderm ; no cleavage cavity was to be observed. 



The rudiments of the former of these two inner layers are develojied 

 from the large dark cell by the separation of a smaller piece, which 

 occupies the hinder pole of the embryo and divides into two cells 

 which become placed symmetrically, one on either side ; they are 

 darker than the ectodermal cells, and their nuclei are larger, so that 

 they altogether resemble in character the primitive mesodermal cells 

 of Unio, Planorbis, Pedicellina, and the Annelids. 



The gastrula arises by epiboly and its free edge is formed by the 

 ectodermal layer ; there is still a single large endodermal cell, which 

 does not become divided for some time, and, even after the commence- 



* ' Claus's Arbeiten,' iii. (1880) p. 1. 



