r
_Coelenterata_. The preceding figures represent two other such forms
in nature, the first locomotive and transitory, the second fixed and
permanent (Figs. 43, 44).

[14] In most vertebrated animals this process of gastrulation has
been more or less superseded by another, which is called
delamination; but it scarcely seems necessary for our present
purposes to describe the latter. For not only does it eventually
lead to the same result as gastrulation--i. e. the converting of the
ovum into a double-walled sac,--but there is good evidence among the
lower Vertebrata of its being preceded by gastrulation; so that,
even as to the higher Vertebrata, embryologists are pretty well
agreed that delamination has been but a later development of, or
possibly improvement upon, gastrulation.

[Illustration: FIG. 41.--Formation of the gastrula of _Amphioxus_.
(After Kowalevsky.) A, wall of the ovum, composed of a single layer
of cells; B, a stage in the process of gastrulation; C, completion
of the process; S, original or segmentation cavity of ovum; _al_,
alimentary cavity of gastrula; _ect_, outer layer of cells; _ent_,
inner layer of cells; _b_, orifice, constituting the mouth in
permanent forms.]

[Illustration: FIG. 42.--Gastrulation. A, Gastrula of a Zoophyte
(_Gastrophysema_). (After Haeckel.) B, Gastrula of a Worm
(_Sagitta_). (After Kowalevsky.) C, Gastrula of an Echinoderm
(_Uraster_). (After A. Agassiz.) D, Gastrula of an Arthropod
(_Nauplius_). (After Haeckel.) E, Gastrula of a Mollusk (_Limnaeus_).
(After Rabl.) F, Gastrula of a Vertebrate (_Amphioxus_). (After
Kowalevsky.) In all, _d_, indicates the intestinal cavity; _o_, the
primitive mouth; _s_, the cleavage-cavity; _i_, the endoderm, or
intestinal layer; _e_, the ectoderm or skin-layer.]

[Illustration: FIG. 43.--Gastrula of a Chalk Sponge. (After Haeckel.)
A, External view. B, Longitudinal section. _g_, digestive cavities;
_o_, mouth; _i_, endoderm; _e_, ectoderm.]

[Illustration: FIG. 44.--_Prophysema primordiale_, an extant
gastraea-form. (After Haeckel.) (A). External view of the whole
animal, attached by its foot to seaweed. (B). Longitudinal section
of the same. The digestive cavity (_d_) opens at its upper end in
the mouth (_m_). Among the cells of the endoderm (_g_) lie amoeboid
egg-cells of large size (_e_). The e