Final answer:
The main difference between rat and snail gastrulation is that rats are deuterostomes where the blastopore becomes the anus, having radial regulative cleavage, and a coelom formed through enterocoely. Snails are protostomes, with the blastopore becoming the mouth, spiral cleavage, and a coelom formed by schizocoely. Both rats and snails develop three germ layers during gastrulation. Option number 4 is correct.
Step-by-step explanation:
During gastrulation, a significant difference between a rat and a snail is that the rat is a deuterostome and the snail is a protostome. This distinction is crucial, as it impacts the development of the digestive system and the body's cavities. In deuterostomes like rats, the blastopore becomes the anus, whereas in protostomes like snails, it develops into the mouth. Furthermore, rats exhibit radial regulative cleavage during early development, and the coelom forms through a process called enterocoely, where pouches from the endodermal tissue form the mesoderm. In contrast, snails, as protostomes, display spiral cleavage, and their coelom forms by schizocoely from mesodermal clumps within the embryo.
Another difference between these two groups is the number of germ layers formed during gastrulation. Rats, being triploblastic, develop three germ layers: the ectoderm, mesoderm, and endoderm. This third layer, the mesoderm, contributes to the formation of various tissues and organs in the body. On the other hand, simpler organisms like snails, which also have three layers, show a more straightforward development. All these distinctions are fundamental aspects of embryonic development that result in the diversity of animal anatomies and life histories.