made at Naples in the winter o/" 1871-72. 87 



Hence the two primitive layers of cells in the embryo mol- 

 lusk have the same origin as in Vermes and Vertebrata ; and, 

 indeed, it would appear that the whole animal series above the 

 Protozoa agree in possessing these two primitive layers at one 

 time of their development. The addition to these of a third, 

 intermediate layer, or mesoderm, is the distinguishing feature 

 of another great branch or stem (Triploblastica), which has as 

 its base the Vermes, and from which diverge the MoUusks, the 

 Arthropods, the Vertebrates, the Echinoderms. That branch 

 which retains but two layers of cells through life, the endoderm 

 and ectoderm, includes the corals, polyps, and sponges (Diplo- 

 blastica). So far biologists seem to have an-ived at very 

 promising results with the germ-layer theory. The great diffi- 

 culty at present lies in the question. Whence does this third layer, 

 or mesoderm, originate? There are a number of conflicting 

 replies to this question, which have yet to be reconciled. 



Development ofTerebella nebulosa. 



An abundant supply of the eggs of this annelid enabled me 

 to follow its development as far as its opacity permits. A 

 delicate chorion forms round the Qgg after segmentation, on the 

 surface of the cleavage-cells, which are densely ciliated. I ob- 

 served that the chorion could be caused to separate from the 

 surface of the cells; and the cilia were then seen to be really 

 processes of the protoplasm of the cells, and to perforate this 

 cuticular exudation, since they did not break off with it, as often 

 happens, but were drawn through it, remaining fixed to the 

 cells. The development of the ciliated tracts, segments, ap- 

 pendages, and tubiparous glands was followed and drawn. 

 The young of this species has no otolithic sac. 



Young Appendicularia furcata. 



Numerous specimens of this most interesting form were ob- 

 tained in February. The recent memoir of Foil has given 

 very full and accurate information on the anatomy of the Ap- 

 pendicular im. 1 have still, however, something to add in this 

 case with regard to the cutaneous glands and the cellular out- 

 growths of the hitegumcnt, and as to the heart. It is ciu-ious 

 that no one has yet drawn attention to the very remarkable 

 fact that the heart '\x\. A. f areata consists of but two cells — that 

 is to say, two nucleated histological units. The small number 

 of histological units which build up the organs of an Appen- 

 dicularia is a very noticeable fact, and is parallelled in the case 

 of the Rotifera. The elaboration, however, of so important an 

 organ as the heart from but two units is quite unexampled. 



