Question

Phylum Platyhelminthes.

whole mount of Planaria aka Dugesia

1. What are the two ways that the gut is similar to the gastrovascular cavities of cnidarian medusae?

2. Compare structures and anatomy of Bdelloura sp to Dugesia sp. How many differences/ similarities can you find?

Answer 1

Phylum platyhelminthes

They belongs to the kingdom animalia. If we broke the term into peices then, play means flat and heminthes means works. They are mostly parasitic, few of which are free living in Sea water and fresh water. They have bilaterally symmetrical body.

Germ layer is triploblastic. Ectoderm is very specialized with distinct epidermis. Coelem is absent hence they are acoelomate. Their digestive system is incomplete or absent. No mouth is present, their opening is directly into the pharynx. They have no true stomach, pharynx opens into a complex intestinal structure. They respirate through simple diffusion of gases through body surface. They have no circulatory system. Reproduction could be done by both asexual and sexual means. Asexual reproduction is by regeneration and fission whereas sexual reproduction is by gametic fusion seen in hermaphrodite species.

Planaria

Planaria is the genus of planarians in the family planariidae. Planaria shows the property of regeneration i.e., if we cut the body of planaria into pieces each piece has the ability to grow into a fully formed individual.

Whole mount of planaria

Whole mount in situ hybridization (WISH) is a powerful tool for visualizing gene expression patterns in specific cells and tissue types. Each model organism shows it's unique set of challenges for achieving robust and reproducible staining with cellular respiration.

planarians have not been amenable to this approach because of two main reasons. They are-

1. First, these organisms have dense mucous layer around their entire bodies thought to facilitate osmotic balance, locomotion, and predator avoidance. The mucous layer also presents a considerable barrier to the exchange if fluids and large molecules needed during WISH.

2. Secondly, organisms are severe millimetres in length, posing​ special challenges with respect to detection efficiency in deep tissues.

Previously, it was found that an alcohol/acid base fixative called Carnoy's could penetrate the mucus barrier and yeild WISH signals in planarians. This method became the quick standard and now is widely used. These are the following strategic step-by-step instructions:

a) killing and mucus removal

b) fixation

c) permeabilization

d) dehydration

E) bleaching

f) riboprobe hydrolysis

g) hybridization solution composition

h) post development treatments

Cniderian medusae

In biology, a Medusa is a form of cniderian in which the body is shaped like an umbrella. The other main body form is the polyp. Medusa vary from Bell shaped to a shape of a thin disk, scarcely convex above and only slightly concave below. The upper surface is called exumbrella and the lower surface is called subumbrella.

Now we have to write two ways that the gut of planaria is similar to the gastrovascular cavities of cniderian medusae. This can be explained by following points:-

1. First of all, we just studied that the planaria has the opening of the gut through the mouth. Planaria are carnivorous bottom feeders and like any other good bottom feeder, their mouth are located at the underside of their body. In the case of Cniderian medusae, the mouth is in the lower surface, which may be partially closed by a membrane extending inward from the margin called the vellum.

2. Secondly, planaria has three branches. Each branch consist of ceca which deleivers nutrients to the body, however this worm has a sac digestive plan i.e., it does not have a separate opening for excretion.

In the case of Cniderian medusae, the digestive cavity consists of gastrovascular cavity and radiating canals which extend towards the margin. These canals may be simple or branching and vary in number from few to many.

The common feature is that both have only one opening for digestion and excretion. That is mouth and opening for excretion are same.