26 THE MOLLUSCA 
An invagination of the ectoderm at the place where the 
blastopore closed, or surrounding the blastopore if it remains open, 
places the digestive cavity (enteron) of the gastrula in communica- 
tion with the exterior. This 
invagination constitutes the 
stomodaeum, from which the 
pharynx and _ oesophagus 
with all their accessory ap- 
paratus, the salivary glands, 
radula, ete., are formed. 
Thus the blastopore, if it 
remains open, does not 
become the mouth of the 
adult.  Puludina, however, 
is an exception, in that 
the whole of the blasto- 
pore remains open and be- 
ae comes the anus (Ray Lan- 
Trochosphere of Patella, sagittal section. 61, blasto- kester), whilst the stomo- 
pore ; me, mesoderm ; sh.g, shell-gland; the twodorsal, Gaeum is formed at the 
ve, and the two ventral ciliated cells are the velar : 
cells. (After Patten.) anterior end of the embryo 
(Fig. 110, F, m). Later, in 
Molluscs in general, the proctodaeum is formed as a very short anal 
invagination, at the posterior end of the original blastoporic groove. 
Its position is generally indicated by the presence of two prominent 
ectodermic cells, and it perforates the posterior part of the archen- 
teron, establishing a com- 
munication between the in- 
testine and the exterior. 
Eetodermic Organs. —The 
embryo, then, has an endo- 
dermic digestive cavity and 
a general ectodermic en- 
velope from which the 
oesophagus and the anal 
invagination are derived. 
But the ectoderm is prin- 
cipally concerned in the 
production of the permanent 
tegumentary organs proper 
Bre obi Se seats) 200); erly Trochosphere of Dreissensia, median sagittal section. 
so called, such as the foot, 1%, blastopore; f, flagellum; in, intestine; p.a.c, 
- 4.” post-anal cilia; sk, shell; ve, velum. (After Meisen- 
the mantle, and the ctenidia, fame.) = ti a 
and those organs which, al- 
though deeply placed, originate from the surface, such as the nervous 
system and the organs of sensation. The ectoderm further gives 
rise to the embryonic locomotory organs which result from a special 
Fic. 13. 
