The Catenulida are an order of turbellaria

Turbellaria

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...

n flatworms.

They are relatively small free-living flatworms, inhabiting freshwater and marine environments. The pharynx

Pharynx

The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

 and intestine are simple in structure, with the latter forming a ciliated sac. They possess two pairs of nerve cords and often a statocyst

Statocyst

The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...

. The gonad

Gonad

The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s are unpaired. Unusually, the male gonopore opens on the dorsal surface of the animal, above the pharynx, while the female reproductive system lacks any of the usual ducts and related structures found in other turbellarians. Asexual reproduction

Asexual reproduction

Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 by paratomy

Paratomy

Paratomy is a form of agammetic asexual reproduction in animals where the organism splits in a plane perpendicular to the antero-posterior axis and the split is preceded by the "pregeneration" of the anterior structures in the posterior portion.The developing organisms have their body axis aligned...

 is common, leading to chains of zooid

Zooid

A zooid or zoöid is a single animal that is part of a colonial animal. The zooids can either be directly connected by tissue or share a common exoskeleton...

s (Latin fem. n. catenula - small chain).