50 



TYPES OF LARVJE. 



Fig. 1 6. 

 Section of Blastula. 



Archicoele. 



by which it rotates freely about the centre. It represents 

 the typical centro- symmetric and 

 monoblastic organism. The layer 

 of cells is called the archiblast and 

 the internal cavity the archicoele (or 

 blastocoele). As the organism is com- 

 pletely centro -symmetric there can 

 be no division of labour between the 

 cells ; hence the blastula represents 



the phyletic stage of a colonial protozoan rather than a true 



metazoan. 



Fig. 17. 

 Section of Morula. 



2. Morula. — The morula differs 

 from the blastula in having the in- 

 ternal cavity filled up with cells, 

 thus forming a solid ball or mul- 

 berry-mass. It is difficult to imagine 

 a living adult organism like a morula, 

 and it is probably a coenogenetic 

 larva. 



2. Diploblastic Larvae. 



3. Gastrula. — This is possibly the most widespread 

 and important larval form. It is typically of a " bell " shape, 

 varying from a " cup " to nearly a sphere or cylinder. Like 



the blastula it is usually a free-swim- 

 ming marine larva. It has two layers 

 of cells — the outer, termed the epi- 

 blast, and the inner, the hypoblast. 

 The internal cavity is termed the 

 archenteron, and its opening to 

 the exterior is called the blastopore. 

 The epiblast cells are usually cili- 

 ated, and the larva is free-swimming, 

 with motion in a spiral direction 

 along the long axis through the blastopore. The gastrula 

 is the typical diploblastic axo-symmetric larva, with physio- 

 logical division of labour between the epiblast and hypoblast, 

 the latter being specially concerned with the function of ali- 

 mentation, the former with those of locomotion, sensation, 

 and excretion. Its body-plan is much the same as that of 

 living Coelenterata. The gastrula is produced from the 



Fig. 18. 

 Section of Gastrula. 



Archenteron 



Epiblast 

 Hypoblast 



Blastopore. 



