Sponges (phylum Porifera) are classified as neither protostomes nor deuterostomes. This classification system, which distinguishes animals based on early embryonic development, simply does not apply to them. Sponges represent the most basal group in the animal kingdom, having diverged from the common ancestor of all other animals before the evolutionary split between protostomes and deuterostomes occurred. Therefore, the developmental mechanisms that define these two major animal clades are entirely absent in the Porifera. Sponges are instead categorized as Parazoa, setting them apart from all other animals, which are collectively known as Eumetazoa.
Defining Protostomes and Deuterostomes
The distinction between protostomes and deuterostomes is founded upon three specific differences in the embryonic development of bilaterally symmetric animals. These two large groups form a major division within the animal kingdom, representing two distinct evolutionary pathways for building a body plan. The most defining difference is the fate of the blastopore, the initial opening that forms in the embryo during a process called gastrulation.
Protostomes
In protostomes, which translates to “first mouth,” the blastopore develops into the organism’s mouth, with the anus forming later from a secondary opening. The coelom, or body cavity, forms through a process called schizocoely, where solid masses of mesoderm tissue split to create the internal space. Protostomes exhibit a spiral and determinate cleavage pattern, meaning the developmental fate of each embryonic cell is fixed very early in development.
Deuterostomes
Conversely, deuterostomes, meaning “second mouth,” have a blastopore that develops into the anus, and the mouth forms secondarily. Coelom formation occurs via enterocoely, in which the coelom arises from outpocketings of the primitive gut, or archenteron. Deuterostome embryos undergo radial and indeterminate cleavage, allowing each early cell to retain the capacity to develop into a complete organism if separated.
The Protostome superphylum includes a large number of invertebrate groups, such as the arthropods, mollusks, and annelids. Deuterostomes include echinoderms and all chordates, which encompasses all vertebrates, including humans. This embryonic distinction is fundamental to understanding the phylogeny of nearly all complex animals.
The Absence of True Tissues in Sponges
The reason sponges fall outside the protostome-deuterostome framework lies in their fundamental lack of true tissues and organ systems. The developmental criteria used to separate protostomes and deuterostomes—blastopore fate, cleavage type, and coelom formation—only apply to animals classified as Eumetazoa, or “true animals.” Eumetazoans are characterized by having true tissues organized from two or three distinct germ layers formed during gastrulation.
Sponges are placed in the subkingdom Parazoa, meaning “beside animals,” because they do not exhibit this tissue-level organization. While sponges are multicellular and have specialized cells like choanocytes and pinacocytes, these cells operate more or less independently. They are not organized into true, functionally coordinated tissue layers separated by a basement membrane.
In the absence of true tissues, sponges do not undergo the complex embryonic process of gastrulation that forms the three germ layers: the ectoderm, mesoderm, and endoderm. This developmental step is a prerequisite for the protostome/deuterostome classification. Without a mesoderm, they cannot form a coelom, and without true germ layers, the concept of a blastopore forming a mouth or an anus is irrelevant to their development.
This simple organization suggests that sponges represent a very early evolutionary stage of multicellular life. Their body consists of a loose federation of cells embedded in a gelatinous matrix called the mesohyl. The lack of these foundational developmental structures makes it impossible to classify them using the protostome or deuterostome labels, which are reserved for animals with bilateral symmetry that are triploblastic.
Unique Developmental Processes in Sponges
Instead of the germ layer formation seen in most other animals, sponge embryogenesis follows developmental pathways unique to the phylum Porifera. After fertilization, the embryo develops internally in the parent sponge into a free-swimming larval stage, which is the primary dispersal mechanism.
Larval Types and Metamorphosis
The two most common larval types are the parenchymula and the amphiblastula. The parenchymula larva is typically solid and entirely covered in flagellated cells, allowing it to swim until it finds a suitable surface to attach. The amphiblastula larva, found in some calcifying sponges, is partially hollow and has distinct front and back halves, with flagellated cells on one end and non-flagellated cells on the other. Both larval forms are short-lived and non-feeding, relying on maternally supplied nutrients.
Once the larva settles on a substrate, it undergoes a dramatic metamorphosis that involves the inversion of cell layers. For instance, in the amphiblastula, the flagellated cells that were on the outside of the larva migrate inward to become the choanocytes, or collar cells, which line the adult sponge’s internal chambers. The cells that were internal in the larva often move to the outside to form the pinacoderm, the outer layer of the adult sponge.
This highly flexible cellular reorganization, where the fate of cells is not rigidly determined early on, contrasts sharply with the fixed developmental patterns of protostomes. The sponge’s method of constructing its body plan—a complex cellular rearrangement rather than a fixed gastrulation process—underscores its basal position in the animal tree of life. The remarkable ability of sponge cells to reaggregate and differentiate highlights a level of developmental plasticity not seen in the more complex protostome or deuterostome clades.