THE MOLLUSCA 25 
is formed from the wall of the enteron, generally as a pair of 
diverticula given off from its middle region (Fig. 116), and com- 
posed exclusively of cells of a fatty nature, while nutrition is effected 
by the absorption of the yolk. 
Formation of the Orifices of the Digestive Tube-—The gastrula 
mouth or blastopore, at a given period of the development at any 
rate, has very often the form of an elongated slit. This condition 
is shown in Patella (Fig. 113, IIL), Bithynia, various Opisthobranchs, 
such as Aplysia and Nudibranchs, in basommatophorous Pulmonates, 
Cyclas, etc. This slit gradually closes up from behind forwards, its 
two margins forming the ventral pedal prominence. In other cases 
this aperture may be more or less elongated, oval in shape, with an 
Fie. 11, ’ ° 
Eggs of Crepidula, showing the origin of the first mesodermic cell. mca, macromeres 3 nies, 
first mesodermic cell; mi, micromeres. (After Conklin.) 
anterior groove running forward as far as the velum, as is seen in 
Paludina. Or again, the blastopore may be circular, and be 
gradually displaced from behind forwards, suggesting a specialised 
condition of the slit-like blastopore which closes in the same 
direction. The linear or circular blastopore is totally closed 
in a large number of cases; such are, Aspidobranchs: Patella, 
Trochus, and Neritina; Pectinibranchs: Bithynia, Nassa, Purpura, 
Nautica, Lamellaria, and Crepidula; Opisthobranchs: Aplysia, 
various Pteropods and Nudibranchs ; Lamellibranchs: Cyrenidae, 
Unionidae, Dreissensia, Teredo. In other forms the blastopore, 
though it may contract so much as to be scarcely visible, remains 
open ; if it is a linear blastopore it is the anterior end that persists. 
Examples are, Chiton; several marine Streptoneura, including 
Vermetus, Fusus, and Heteropods ; Pulmonata ; Dentalinin ; Nucula ; 
Ostraca. 
