160 E, RAY LANKESTER. 
paper on Limneus), in deference to Professor Haeckel’s 
terminology, spoken of as the ‘‘ Gastrula-mouth,” but 
Soe) P&E 2 
which I propose no longer to speak of as ‘‘ mouth.” The di- 
ploblastic Planula of Paludina does not, then, arise by the 
formation of an endoderm by ‘‘ delamination,” as is the case 
in some embryonic histories (e. g. the Hydroid Polyps), but 
it arises by ‘ invagination.” 
In Fig. 2, we see the embryo in a stage considerabl 
fo ) d 
beyond this. The two primitive layers are as strongly 
marked as in the younger stage, but in addition we have 
fusiform and detached corpuscles disposed in the space 
(primitive body-cayity) between endoderm and ectoderm. 
These ‘‘ mesoblastic ” cells take their origin, I believe, partly 
from ectodermal, partly from endodermal parents. The velum 
(v) is now well developed and of large size relatively to 
the rest of the embryo. It subsequently becomes a curious 
little cap-like eminence above the mouth.! In the middle 
line the pharynx (p h) has appeared asa distinct involution, 
the widely open orifice of which is the mouth. It has not 
1 Leydig has well described the general form of the embryo at different 
stages. He has not, however, seen the “‘shell-gland,” which I have detected 
in Paludina, thus extending the list of molluscan embryos provided with 
this problematical organ. I discovered this organ in Pisidium and the 
Nudibranchs in 1871. In Paludina the young shell has a chitinous boss or 
handle which fits into the ‘ shell-gland,” as in Neritina. 
