THE MOLLUSC A 25 



is formed from the wall of the enteron, generally as a pair of 

 diveVticula 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, III), Bithynia, various Opisthobranchs, 

 such as Aplysia and Nudibranchs, in basommatophorous Pulmonates, 

 Cydas, 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 



Km. 11. 



Eggs of Crepidula, showing tlin origin of the Hrst mesolermic cell. , macromeres ; ies f 

 first niesodermic cell ; mi, inicroiueres. (After (Joiiklin.) 



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, 

 Natica, Lamellaria, and Crcpidida ; Opisthobranchs : Aplysia, 

 various Pteropods and Nudibranchs ; Lamellibranchs : Cyrenidae, 

 Unionidae, Dreissemia, 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 

 VcrmetuS) Fusus, and Heteropods ; Pulmonata ; Dentalium ; Nucula ; 

 Ostraea. 



