Fig.30 Gastrula of a very simple primitive-gut animal or gastræad. Fig. 31 Gastrula of a worm. Fig. 32 Gastrula of an echinoderm. Fig. 33 Gastrula of an arthropod. Fig. 34 Gastrula of a mollusc. Fig. 35 Gastrula of a vertebrate. Fig. 30 (_A_)—Gastrula of a very simple primitive-gut animal or gastræad (gastrophysema). (_Haeckel._) Fig. 31 (_B_)—Gastrula of a worm (_Sagitta_). (From _Kowalevsky._) Fig. 32 (_C_)—Gastrula of an echinoderm (star-fish, _Uraster_), not completely folded in (depula). (From _Alexander Agassiz._) Fig. 33 (_D_)—Gastrula of an arthropod (primitive crab, _Nauplius_) (as 32). Fig. 34 (_E_)—Gastrula of a mollusc (pond-snail, _Linnæus_). (From _Karl Rabl._) Fig. 35 (_F_)—Gastrula of a vertebrate (lancelet, _Amphioxus_). (From _Kowalevsky._) (Front view.) In each figure _d_ is the primitive-gut cavity, _o_ primitive mouth, _s_ segmentation-cavity, _i_ entoderm (gut-layer), _e_ ectoderm (skin layer).
In view of this extraordinary significance of the gastrula, we must make a very careful study of its original structure. As a rule, the typical gastrula is very small, being invisible to the naked eye, or at the most only visible as a fine point under very favourable conditions, and measuring generally 1/500 to 1/250 of an inch (less frequently 1/50 inch, or even more) in diameter. In shape it is usually like a roundish drinking-cup. Sometimes it is rather oval, at other times more ellipsoid or spindle-shaped; in some cases it is half round, or even almost round, and in others lengthened out, or almost cylindrical.
I give the name of primitive gut (_progaster_) and primitive mouth (_prostoma_) to the internal cavity of the gastrula-body and its opening; because this cavity is the first rudiment of the digestive cavity of the organism, and the opening originally served to take food into it. Naturally, the primitive gut and mouth change very considerably afterwards in the various classes of animals. In most of the cnidaria and many of the annelids (worm-like animals) they remain unchanged throughout life. But in most of the higher animals, and so in the vertebrates, only the larger central part of the later alimentary canal develops from the primitive gut; the later mouth is a fresh development, the primitive mouth disappearing or changing into the anus. We must therefore distinguish carefully between the primitive gut and mouth of the gastrula and the later alimentary canal and mouth of the fully developed vertebrate.[19]