primitive multicellular animals. Their body is built of uneven, porous mass of
cells connected with channels of different direction. These mostly
immovable sea animals live on the sea and ocean bottom attached to
different subaquatic objects. According to the shape and colour they are
extremely varied. As to the size, they from range millimeters to a meter and
more. On their surface there are a lot of pores from where their Latin name
Porifera, i.e. porous animals.
The reproduction of sea fungi is realized by both asexual and sexual
way. They posses large possibilities to regenerate from single cells. If a
part of their body is ground to individual cells and then filtered, the cells
preserve their viability, amoeba-like motion, gather together in small
groups, form shapeless associations of cells which after several days
become new small fungi.
The hydra (Hydra vulgaris) — see Fig. 4–3, is a typical fresh-water
animal, which place is at the beginning of the evolution of animal
multicellular organisms. Firstly it is examined under the microscope by van
Leeuwenhoek. It is a small beast of prey feeding with infusoria, plankton
crabs, small fishes, small worms, etc. It is known with its high ability to
regenerate as from separate parts, as well as from single cells. Dissected
in half, it rapidly regenerates the missing part. The astonishing thing in this
case is that on the anterior end of the piece always a “head” with tentacles
grows, and on the posterior one — pedicel. The cells in the middle of the
body reproduce more intensively, from where they move in opposite
directions. From single cells daughter hydras are developed by budding.
Except asexually (by budding) hydra also reproduces sexually, developing
male and female sexual cells (gonads). Here, there is a well-expressed
specialization and differentiation of the cells.
The brief survey of the organization of some of the lower multicellular
plant and animal organisms gives good reason to accept that their
formation is a result of integrating of individual cells in common biosystems.
As a result of the cell interactions, covering layers and membranes (like
those of independently existed unicellular organisms) arise. They protect
the cells from outer influences, take or secrete outwards different chemical
elements and substances, thus creating the necessary favourable
conditions for a normal realization of life processes in them. The
consolidation of cells in a united organism gives rise to the necessity of
specializing their functions. There appear different types of cells, tissues
and organs, and in the course of evolution not only their morphological
diversity increases, but also their strict functional specificity. Studying this
“evolutionary heritage” is a subject of separate branches of biology —
anatomy, histology, etc.