* Trilobozoa * Porifera * Placozoa * Vendobionta * Cnidaria

* Myxozoa * Ctenophora * Acoelomorpha * Orthonectida * Dicyemida * Gnathostomulida * Cycliophora * Micrognathozoa * Rotifera * Acanthocephala * Gastrotricha

* Platyhelminthes * Phoronida * Brachiopoda* Bryozoa * Entoprocta * Nemertea * Sipuncula * Mollusca * Hyolitha * Echiura * Annelida * Chaetognatha * Nematoda * Nematomorpha * Priapulida * Kinorhyncha * Loricifera * Onychophora * Tardigrada * Arthropoda * Vetulicolia * Xenoturbellida * Echinodermata * Hemichordata* Chordata Sometimes, are considered invertebrates also Kingdom Protozoa (not true animals), which phyla are:

* Amoebozoa * Choanozoa * Cercozoa * Foraminifera * Radiozoa * Apusozoa * Loukozoa * Percolozoa * Euglenozoa * Metamonada * Myzozoa * Ciliophora * Heliozoa

The Major phylums in the animal kingdom are as follows.

INVERTEBRATES

Phylum: Porifera

Phylum: Coelenterata/ Cnidaria

Phylum: Platyhelminthes

Phylum: Nematehelminthes/ Aschelminthese

Phylum: Annelida

Phylum: Arthropoda

Phylum: Mollusca

Phylum: Echinodermata.

The phylum protozoa is now not treated as animal phylum and it is included under a separate kingdom called PROTISTA.

There are animals which are intermediate between invertebrates and vertebrates and are called protochordates.

PROTOCHORDATES

Phylum: Hemichordata

phylum: VERTEBRATA, under this phylum there are six classes

class: Pisces

class: Amphibians

class: Reptilia

class: Aves

class: Mammalia

All the above are mojor phylums, along with these phylums, there are many minor phylums.

Any aquatic invertebrate could be described that way. Perhaps the term "marine invertebrate" is what you are looking for.

Invertebrate sea life includes the following phyla:

• Acoela;
• Annelida, (polychaetes and sea leeches);
• Brachiopoda;
• Bryozoa, also known as moss animals or sea mats;
• Chaetognatha;
• Cephalochordata;
• Cnidaria, such as jellyfish, sea anemones, and corals;
• Crustacea, including lobsters, crabs, shrimp, crayfish, barnacles, hermit crabs, mantis shrimps, and copepods;
• Ctenophora, also known as comb jellies;
• Echinodermata, including sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, crinoids, and sea daisies;
• Echiura;
• Gnathostomulids;
• Gastrotricha;
• Hemichordata;
• Kamptozoa;
• Kinorhyncha;
• Loricifera;
• Merostomata; also know known as horseshoe crabs;
• Mollusca, including shellfish, squid, octopus, whelks, Nautilus, cuttlefish, nudibranchs, scallops, sea snails, Aplacophora, Caudofoveata, Monoplacophora, Polyplacophora, and Scaphopoda;
• Myzostomida;
• Nemertinea (ribbon worms);
• Orthonectida;
• Phoronida;
• Placozoa;
• Porifera (sponges);
• Priapulida;
• Pycnogonida (sea spiders);
• Sipunculida;
• Tunicata, also known as sea squirts;
• Some flatworms of the classes Turbellaria and Monogenea;
• Xenoturbella;
• Xiphosura.

I notice that this was originally posted in "aquatic mammals". All mammals are vertebrates so none of the aquatic mammals would lack a backbone.