Pseudocoelomates (Rotifers, Nematodes and others)



1) Body cavity designated as pseudocoel.

2) Simple arrangement of tissues and organs within pseudocoelomic cavity.

3) Triploblastic: three germ layers.

4) Ectoderm basically same as in all three, giving rise to epidermis (some specialized structures present).

5) Endoderm basically same all three digestive system on tract derived from endoderm.


Key Differences

1) Complexity, arrangement, and function of mesoderm.

a) Acoelomates: mesoderm is in "continuous" contact with endoderm and ectoderm; specific interaction with digestive portion of organism: NO BODY CAVITY.

b) Pseudocoelomates: mesoderm "discontinuous" with regard to endoderm but not contact with ectoderm.

c) Coelomate: also called eucoelomate; body cavity derived and embedded within mesoderm itself; mesoderm is complex and very specialized with two specific tissues derived form mesodermal germ layer.

1) Mesentary: mesodermal tissue specialized for suspending and attaching to specific digestive system organs; divides eucoelom into distinct chambers throughout body, giving rise to internal segmentation (even early embryonic stage) and external segmentation. Internal segmentation is primarily due to lateral partitioning by mesentary; some longitudinal partitioning occurs within eucoelomate by longitudinal arrangement of mesentary tissue.

2) Peritoneum: internal, mesodermal lining of eucoelom.


Functional Aspects

1) Pseudocoelom: provides anchor matrix of muscle fibers, etc., derived form mesoderm for suspension of digestive system; fluid-filled cavity, therefore hydrostatic pressure provides support for overall body structure and shape of organism; mostly only mesodermally. Derived, longitudinal muscles for longitudinal coiling of organisms (roundworms of Phylum Nematoda).

2) Eucoelom: very good anchoring and suspension internal organs by mesentary, providing support, circulation is crude, open circulatory system of arthropods, excretion and osmoregulation (some spp.), respiration (gas exchange) in some aquatic spp. No digestion occurs within eucolom; only occurs within digestive system suspended within eucoelom.


Pseudocoelomate Phyla:

1) Phylum Rotifera (rotifers) - small, microscopic multicellular aquatic.

2) Phylum Nematoda (roundworms) - common organisms. vinegar eels & nematodes.

3) Phylum Entoprocta - many microscopic hydra-like pseudocoelomates. lots specialized internal structure, especially within eocoelom. mostly marine organisms.


Focus on Phylum Rotifera - having, bearing roti = wheel

External structures

Organisms bearing some type of specialized structure: corona (anterior, wheel-like structure with many cilia for locomotion and feeding. Also close to mouth.

Internal structures

1) Digestive system is complete with mouth associated with a specialized external structure (corona). pharynx is connecting mouth to digestive region and also with gastric gland (stomach-like organ which secretes acids and enzymes for chemical and enzymatic digestion of food particles). Intestine and anus present.