Pseudocoelomate Animals

I. These animals have an unlined body cavity called a pseudocoel.

   A. A space between the gut and the mesodermal components of the body wall.

   B. Functions

     1. Distribution of nutrients

     2. Removal of wastes

     3. Storage space for maturing gametes

     4. Serves as a hydrostatic skeleton

II. Phylum Rotifera, members of this group are often called wheel animals because they have cilia at their anterior ends that resemble wheels in motion.

   A. Rotifers are mostly freshwater, some marine, and some parasitic.

   B. Their body form is cylindrical with a shell-like cuticle.

   C. The foot (posterior end), has one to four toes that secrete a sticky substance from the pedal glands for attachment.

   D. Food is swept into the mouth by cilia and on to the pharynx.

   E. The lower end of the pharynx is the mastax, equipped with an intricate set of jaws that are chitin-like in nature and grind the planktonic forms that are rotifer food.

   F. The esophagus leads from the mastax to a stomach or stomach and short intestine, depending on the species.

   G. Undigested material passes through the cloaca, (Latin for sewer).

   H. Flame cells are spaced at intervals along two long tubes which open into a bladder that contracts periodically and forces the contents out of the cloacal opening.

   I. Rotifers are dioecious.  Males are unknown in some species.

     1. Females may produce eggs that have not undergone meiosis and these hatch into females only.  These eggs are called amictic and they cannot be fertilized.  These are the only eggs that are produced by those species whose members are all female and reproduce through parthenogenesis.   

     2. Other species may produce two kinds of eggs.

       a. Amictic eggs – see above.

       b. Mictic eggs have undergone meiosis and, if unfertilized, develop quickly into males.  If they are fertilized, they develop a hard chitin-like case and are dormant for a time before hatching into females.  These eggs can withstand desiccation and allow for life in temporary ponds.

III. Phylum Nematoda, the roundworms.

   A. General characteristics

     1. Body is bilaterally symmetrical.

     2. Body is covered with a cuticle.

     3. Muscles in the body wall run in longitudinal direction only.

     4. No flame cells, excretion is by glands or excretory canals.

     5. Male reproductive tract opens to the rectum to form a cloaca; female tract has a separate opening called a gonopore.

   B. Ascaris lumbricoides is a common parasite in pigs’ and humans’ intestine.

     1. These worms are dioecious with sexual dimorphism, i.e. the male and female are different.  The males are smaller than the much larger females who may be up to 25 cm. long and 6 mm wide.

     2. The mouth is anterior and surrounded by one dorsal and two lateroventral lips.

     3. Eggs are voided with feces from the host animal and hatch in the intestine after contaminated soil, water, etc. is ingested.

       a. Larvae enter the veins or lymphatic system and are eventually carried to the right side of the heart.

       b. From there they travel to the lungs to the esophagus to the stomach to the intestine, where they live and mature.

       c. The trip lasts about 10 days, the worms mature in about 2.5 months.