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It has since been revealed that even if galeaspidans have just one external opening, it has two internal nasal organs. [5] [6] After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 420 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian ...
Ceratium dinoflagellates may reproduce sexually (two parent cells) or asexually (one parent cell) . [4] In asexual reproduction, the pellicle (shell) pulls apart and exposes the naked cell. [ 2 ] The cell then increases in size and divides, creating 4–8 daughter cells, each with two flagella. [ 2 ]
Pacific sea nettles, Chrysaora fuscescens. Cnidaria (/ n ɪ ˈ d ɛər i ə, n aɪ-/ nih-DAIR-ee-ə, NY-) [4] is a phylum under kingdom Animalia containing over 11,000 species [5] of aquatic invertebrates found both in fresh water and marine environments (predominantly the latter), including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites.
Worms (Old English for serpent) typically have long cylindrical tube-like bodies and no limbs. Marine worms vary in size from microscopic to over 1 metre (3.3 ft) in length for some marine polychaete worms ( bristle worms ) [ 42 ] and up to 58 metres (190 ft) for the marine nemertean worm ( bootlace worm ). [ 43 ]
Agnatha (/ ˈ æ ɡ n ə θ ə, æ ɡ ˈ n eɪ θ ə /; [3] from Ancient Greek ἀ-(a-) 'without' and γνάθος (gnáthos) 'jaws') is a paraphyletic infraphylum [4] of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct (conodonts, anaspids, and ostracoderms, among others).
It has a polymorphic life cycle, ranging from free-living cells to large colonies. [196] It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms. [197] As a result, Phaeocystis is an important contributor to the marine carbon [198] and sulfur cycles ...
Distinctively, these fish have no jaws. [15] Cartilaginous fish such as sharks also have simple, and presumably primitive, skull structures. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle.
A shed carapace of a lady crab, part of the hard exoskeleton Body structure of a typical crustacean – krill. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, [5] the pereon or thorax, [6] and the pleon or abdomen. [7]